Merge branch 'main' into main

Bump github/codeql-action from 3.26.11 to 3.26.12

.cirrus.yml

@@ -14,12 +14,10 @@ freebsd_task:
         image_family: freebsd-14-0
 
   env:
-    TEST_ALLOW_SEND: 0
     TEST_SKIP_SENDMAIL: 1
 
   pkginstall_script:
-    - pkg update -f
     - pkg install -y go
 
   test_script:
-    - go test -v -race -cover -shuffle=on ./...
+    - go test -race -cover -shuffle=on ./...

.github/workflows/codecov.yml

@@ -10,7 +10,7 @@ on:
     paths:
       - '**.go'
       - 'go.*'
-      - '.github/**'
+      - '.github/workflows/codecov.yml'
       - 'codecov.yml'
   pull_request:
     branches:
@@ -18,7 +18,7 @@ on:
     paths:
       - '**.go'
       - 'go.*'
-      - '.github/**'
+      - '.github/workflows/codecov.yml'
       - 'codecov.yml'
 env:
   TEST_HOST: ${{ secrets.TEST_HOST }}
@@ -27,6 +27,10 @@ env:
   TEST_SMTPAUTH_USER: ${{ secrets.TEST_USER }}
   TEST_SMTPAUTH_PASS: ${{ secrets.TEST_PASS }}
   TEST_SMTPAUTH_TYPE: "LOGIN"
+  TEST_ONLINE_SCRAM: "1"
+  TEST_HOST_SCRAM: ${{ secrets.TEST_HOST_SCRAM }}
+  TEST_USER_SCRAM: ${{ secrets.TEST_USER_SCRAM }}
+  TEST_PASS_SCRAM: ${{ secrets.TEST_PASS_SCRAM }}
 permissions:
   contents: read
 
@@ -36,10 +40,10 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        go: ['1.20', '1.21', '1.22', '1.23']
+        go: ['1.23']
     steps:
     - name: Harden Runner
-      uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+      uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1
       with:
         egress-policy: audit
 
@@ -55,9 +59,9 @@ jobs:
         sudo apt-get -y install sendmail; which sendmail
     - name: Run Tests
       run: |
-        go test -v -race --coverprofile=coverage.coverprofile --covermode=atomic ./...
+        go test -race --coverprofile=coverage.coverprofile --covermode=atomic ./...
     - name: Upload coverage to Codecov
       if: success() && matrix.go == '1.23' && matrix.os == 'ubuntu-latest'
-      uses: codecov/codecov-action@e28ff129e5465c2c0dcc6f003fc735cb6ae0c673 # v4.5.0
+      uses: codecov/codecov-action@b9fd7d16f6d7d1b5d2bec1a2887e65ceed900238 # v4.6.0
       with:
         token: ${{ secrets.CODECOV_TOKEN }} # not required for public repos

.github/workflows/codeql-analysis.yml

@@ -45,7 +45,7 @@ jobs:
 
     steps:
     - name: Harden Runner
-      uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+      uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1
       with:
         egress-policy: audit
 
@@ -54,7 +54,7 @@ jobs:
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@4dd16135b69a43b6c8efb853346f8437d92d3c93 # v3.26.6
+      uses: github/codeql-action/init@c36620d31ac7c881962c3d9dd939c40ec9434f2b # v3.26.12
       with:
         languages: ${{ matrix.language }}
         # If you wish to specify custom queries, you can do so here or in a config file.
@@ -65,7 +65,7 @@ jobs:
     # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
     # If this step fails, then you should remove it and run the build manually (see below)
     - name: Autobuild
-      uses: github/codeql-action/autobuild@4dd16135b69a43b6c8efb853346f8437d92d3c93 # v3.26.6
+      uses: github/codeql-action/autobuild@c36620d31ac7c881962c3d9dd939c40ec9434f2b # v3.26.12
 
     # ℹ️ Command-line programs to run using the OS shell.
     # 📚 https://git.io/JvXDl
@@ -79,4 +79,4 @@ jobs:
     #   make release
 
     - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@4dd16135b69a43b6c8efb853346f8437d92d3c93 # v3.26.6
+      uses: github/codeql-action/analyze@c36620d31ac7c881962c3d9dd939c40ec9434f2b # v3.26.12

.github/workflows/dependency-review.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+        uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1
         with:
           egress-policy: audit
 

.github/workflows/golangci-lint.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+        uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1
         with:
           egress-policy: audit
 
@@ -29,7 +29,7 @@ jobs:
           go-version: '1.23'
       - uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # v3.6.0
       - name: golangci-lint
-        uses: golangci/golangci-lint-action@aaa42aa0628b4ae2578232a66b541047968fac86 # v6.1.0
+        uses: golangci/golangci-lint-action@971e284b6050e8a5849b72094c50ab08da042db8 # v6.1.1
         with:
           # Optional: version of golangci-lint to use in form of v1.2 or v1.2.3 or `latest` to use the latest version
           version: latest

.github/workflows/govulncheck.yml

@@ -14,8 +14,8 @@ jobs:
     runs-on: ubuntu-latest
     steps: 
     - name: Harden Runner
-      uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+      uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1
       with:
         egress-policy: audit
     - name: Run govulncheck
-      uses: golang/govulncheck-action@dd0578b371c987f96d1185abb54344b44352bd58 # v1.0.3
+      uses: golang/govulncheck-action@b625fbe08f3bccbe446d94fbf87fcc875a4f50ee # v1.0.4

.github/workflows/offline-tests.yml

@@ -0,0 +1,45 @@
+# SPDX-FileCopyrightText: 2022 Winni Neessen <winni@neessen.dev>
+#
+# SPDX-License-Identifier: CC0-1.0
+
+name: Offline tests workflow
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - '**.go'
+      - 'go.*'
+      - '.github/workflows/offline-tests.yml'
+  pull_request:
+    branches:
+      - main
+    paths:
+      - '**.go'
+      - 'go.*'
+      - '.github/workflows/offline-tests.yml'
+permissions:
+  contents: read
+
+jobs:
+  run:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        go: ['1.19', '1.20', '1.21', '1.22', '1.23']
+    steps:
+    - name: Harden Runner
+      uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1
+      with:
+        egress-policy: audit
+
+    - name: Checkout Code
+      uses: actions/checkout@61b9e3751b92087fd0b06925ba6dd6314e06f089 # master
+    - name: Setup go
+      uses: actions/setup-go@0a12ed9d6a96ab950c8f026ed9f722fe0da7ef32 # v5.0.2
+      with:
+        go-version: ${{ matrix.go }}
+    - name: Run Tests
+      run: |
+        go test -race -shuffle=on ./...

.github/workflows/reuse.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     steps: 
     - name: Harden Runner
-      uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+      uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1
       with:
         egress-policy: audit
 

.github/workflows/scorecards.yml

@@ -35,7 +35,7 @@ jobs:
 
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+        uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1
         with:
           egress-policy: audit
 
@@ -67,7 +67,7 @@ jobs:
       # Upload the results as artifacts (optional). Commenting out will disable uploads of run results in SARIF
       # format to the repository Actions tab.
       - name: "Upload artifact"
-        uses: actions/upload-artifact@50769540e7f4bd5e21e526ee35c689e35e0d6874 # v4.4.0
+        uses: actions/upload-artifact@604373da6381bf24206979c74d06a550515601b9 # v4.4.1
         with:
           name: SARIF file
           path: results.sarif
@@ -75,6 +75,6 @@ jobs:
 
       # Upload the results to GitHub's code scanning dashboard.
       - name: "Upload to code-scanning"
-        uses: github/codeql-action/upload-sarif@4dd16135b69a43b6c8efb853346f8437d92d3c93 # v3.26.6
+        uses: github/codeql-action/upload-sarif@c36620d31ac7c881962c3d9dd939c40ec9434f2b # v3.26.12
         with:
           sarif_file: results.sarif

.github/workflows/sonarqube.yml

@@ -10,24 +10,25 @@ permissions:
 on:
   push:
     branches:
-      - main # or the name of your main branch
+      - main
+    paths:
+      - '**.go'
+      - 'go.*'
+      - '.github/workflows/sonarqube.yml'
   pull_request:
     branches:
-      - main # or the name of your main branch
+      - main
-env:
+    paths:
-  TEST_HOST: ${{ secrets.TEST_HOST }}
+      - '**.go'
-  TEST_FROM: ${{ secrets.TEST_USER }}
+      - 'go.*'
-  TEST_ALLOW_SEND: "1"
+      - '.github/workflows/sonarqube.yml'
-  TEST_SMTPAUTH_USER: ${{ secrets.TEST_USER }}
-  TEST_SMTPAUTH_PASS: ${{ secrets.TEST_PASS }}
-  TEST_SMTPAUTH_TYPE: "LOGIN"
 jobs:
   build:
     name: Build
     runs-on: ubuntu-latest
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+        uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1
         with:
           egress-policy: audit
 
@@ -38,13 +39,13 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@0a12ed9d6a96ab950c8f026ed9f722fe0da7ef32 # v5.0.2
         with:
-          go-version: '1.23.x'
+          go-version: '1.23'
           
       - name: Run unit Tests
         run: |
-          go test -v -race --coverprofile=./cov.out ./...
+          go test -shuffle=on -race --coverprofile=./cov.out ./...
 
-      - uses: sonarsource/sonarqube-scan-action@0c0f3958d90fc466625f1d1af1f47bddd4cc6bd1 # master
+      - uses: sonarsource/sonarqube-scan-action@884b79409bbd464b2a59edc326a4b77dc56b2195 # master
         env:
           SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}
           SONAR_HOST_URL: ${{ secrets.SONAR_HOST_URL }}

.reuse/dep5

@@ -1,10 +0,0 @@
-Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
-Upstream-Name: go-mail
-Upstream-Contact: Winni Neessen <winni@neessen.dev>
-Source: https://github.com/wneessen/go-mail
-
-# Sample paragraph, commented out:
-#
-# Files: src/*
-# Copyright: $YEAR $NAME <$CONTACT>
-# License: ...

README.md

@@ -18,40 +18,41 @@ SPDX-License-Identifier: CC0-1.0
 
 <p align="center"><img src="./assets/gopher2.svg" width="250" alt="go-mail logo"/></p>
 
-The main idea of this library was to provide a simple interface to sending mails for
+The main idea of this library was to provide a simple interface for sending mails to
 my [JS-Mailer](https://github.com/wneessen/js-mailer) project. It quickly evolved into a full-fledged mail library.
 
-go-mail follows idiomatic Go style and best practice. It's only dependency is the Go Standard Library. It combines a lot
+go-mail follows idiomatic Go style and best practice. It has a small dependency footprint by mainly relying on the
-of functionality from the standard library to give easy and convenient access to mail and SMTP related tasks.
+Go Standard Library and the Go extended packages. It combines a lot of functionality from the standard library to 
+give easy and convenient access to mail and SMTP related tasks.
 
-Parts of this library (especially some parts of [msgwriter.go](msgwriter.go)) have been forked/ported from the
+In the early days, parts of this library (especially some parts of [msgwriter.go](msgwriter.go)) had been 
-[go-mail/mail](https://github.com/go-mail/mail) respectively [go-gomail/gomail](https://github.com/go-gomail/gomail)
+forked/ported from [go-mail/mail](https://github.com/go-mail/mail) and respectively [go-gomail/gomail](https://github.com/go-gomail/gomail). Today
-which both seems to not be maintained anymore.
+most of the ported code has been refactored.
 
-The smtp package of go-mail is forked from the original Go stdlib's `net/smtp` and then extended by the go-mail
+The `smtp` package of go-mail has been forked from the original Go stdlib's `net/smtp` package and has then been extended 
-team.
+by the go-mail team to fit the packages needs (more SMTP Auth methods, logging, concurrency-safety, etc.).
 
 ## Features
 
-Some of the features of this library:
+Here are some highlights of go-mail's featureset:
 
-* [X] Only Standard Library dependant
+* [X] Very small dependency footprint (mainly Go Stdlib and Go extended packages)
 * [X] Modern, idiomatic Go
 * [X] Sane and secure defaults
 * [X] Explicit SSL/TLS support
 * [X] Implicit StartTLS support with different policies
 * [X] Makes use of contexts for a better control flow and timeout/cancelation handling
-* [X] SMTP Auth support (LOGIN, PLAIN, CRAM-MD, XOAUTH2)
+* [X] SMTP Auth support (LOGIN, PLAIN, CRAM-MD, XOAUTH2, SCRAM-SHA-1(-PLUS), SCRAM-SHA-256(-PLUS))
 * [X] RFC5322 compliant mail address validation
 * [X] Support for common mail header field generation (Message-ID, Date, Bulk-Precedence, Priority, etc.)
-* [X] Reusing the same SMTP connection to send multiple mails
+* [X] Concurrency-safe reusing the same SMTP connection to send multiple mails
 * [X] Support for attachments and inline embeds (from file system, `io.Reader` or `embed.FS`)
 * [X] Support for different encodings
 * [X] Middleware support for 3rd-party libraries to alter mail messages
 * [X] Support sending mails via a local sendmail command
 * [X] Support for requestng MDNs (RFC 8098) and DSNs (RFC 1891)
 * [X] DKIM signature support via [go-mail-middlware](https://github.com/wneessen/go-mail-middleware)
-* [X] Message object satisfies `io.WriteTo` and `io.Reader` interfaces
+* [X] Message object satisfies `io.WriterTo` and `io.Reader` interfaces
 * [X] Support for Go's `html/template` and `text/template` (as message body, alternative part or attachment/emebed)
 * [X] Output to file support which allows storing mail messages as e. g. `.eml` files to disk to open them in a MUA
 * [X] Debug logging of SMTP traffic
@@ -76,7 +77,8 @@ We guarantee that go-mail will always support the last four releases of Go. With
 the user a timeframe of two years to update to the next or even the latest version of Go.
 
 ## Support
-We have a support and general discussion channel on Discord. Find us at: [#go-mail](https://discord.gg/dbfQyC4s)
+We have a support and general discussion channel on Discord. Find us at: [#go-mail](https://discord.gg/dbfQyC4s) alternatively find us
+on the [Gophers Slack](https://gophers.slack.com) in #go-mail
 
 ## Middleware
 The goal of go-mail is to keep it free from 3rd party dependencies and only focus on things a mail library should
@@ -99,15 +101,18 @@ We provide example code in both our GoDocs as well as on our official Website (s
 check out our [Getting started](https://go-mail.dev/getting-started/introduction/) guide.
 
 ## Authors/Contributors
-go-mail was initially authored and developed by [Winni Neessen](https://github.com/wneessen/).
+go-mail was initially created and developed by [Winni Neessen](https://github.com/wneessen/), but over time a lot of amazing people 
+contributed ot the project. Big thanks to all of them for improving the go-mail project (be it writing code, testing
+code, reviewing code, writing documenation or helping to translate the website):
 
-Big thanks to the following people, for contributing to the go-mail project (either in form of code or by 
+<a href="https://github.com/wneessen/go-mail/graphs/contributors">
-reviewing code, writing documenation or helping to translate the website):
+  <img src="https://contrib.rocks/image?repo=wneessen/go-mail" />
-* [Christian Vette](https://github.com/cvette)
+</a>
-* [Dhia Gharsallaoui](https://github.com/dhia-gharsallaoui)
+
-* [inliquid](https://github.com/inliquid) 
+A huge thank you also goes to [Maria Letta](https://github.com/MariaLetta) for designing our super cool go-mail logo!
-* [iwittkau](https://github.com/iwittkau)
+
-* [James Elliott](https://github.com/james-d-elliott)
+## Sponsors
-* [Maria Letta](https://github.com/MariaLetta) (designed the go-mail logo)
+We sincerely thank our amazing sponsors for their generous support! Your contributions do not go unnoticed and helps
-* [Nicola Murino](https://github.com/drakkan)
+keeping up the project!
-* [sters](https://github.com/sters)
+
+* [kolaente](https://github.com/kolaente)

REUSE.toml

@@ -0,0 +1,9 @@
+# SPDX-FileCopyrightText: Copyright (c) 2022-2024 The go-mail Authors
+#
+# SPDX-License-Identifier: MIT
+
+version = 1
+SPDX-PackageName = "go-mail"
+SPDX-PackageSupplier = "Winni Neessen <winni@neessen.dev>"
+SPDX-PackageDownloadLocation = "https://github.com/wneessen/go-mail"
+annotations = []

auth.go

@@ -6,41 +6,131 @@ package mail
 
 import "errors"
 
-// SMTPAuthType represents a string to any SMTP AUTH type
+// SMTPAuthType is a type wrapper for a string type. It represents the type of SMTP authentication
+// mechanism to be used.
 type SMTPAuthType string
 
-// Supported SMTP AUTH types
 const (
-	// SMTPAuthCramMD5 is the "CRAM-MD5" SASL authentication mechanism as described in RFC 4954
+	// SMTPAuthCramMD5 is the "CRAM-MD5" SASL authentication mechanism as described in RFC 4954.
+	// https://datatracker.ietf.org/doc/html/rfc4954/
+	//
+	// CRAM-MD5 is not secure by modern standards. The vulnerabilities of MD5 and the lack of
+	// advanced security features make it inappropriate for protecting sensitive communications
+	// today.
+	//
+	// It was recommended to deprecate the standard in 20 November 2008. As an alternative it
+	// recommends e.g. SCRAM or SASL Plain protected by TLS instead.
+	//
+	// https://datatracker.ietf.org/doc/html/draft-ietf-sasl-crammd5-to-historic-00.html
 	SMTPAuthCramMD5 SMTPAuthType = "CRAM-MD5"
 
-	// SMTPAuthLogin is the "LOGIN" SASL authentication mechanism
+	// SMTPAuthCustom is a custom SMTP AUTH mechanism provided by the user. If a user provides
+	// a custom smtp.Auth function to the Client, the Client will its smtpAuthType to this type.
+	//
+	// Do not use this SMTPAuthType without setting a custom smtp.Auth function on the Client.
+	SMTPAuthCustom SMTPAuthType = "CUSTOM"
+
+	// SMTPAuthLogin is the "LOGIN" SASL authentication mechanism. This authentication mechanism
+	// does not have an official RFC that could be followed. There is a spec by Microsoft and an
+	// IETF draft. The IETF draft is more lax than the MS spec, therefore we follow the I-D, which
+	// automatically matches the MS spec.
+	//
+	// Since the "LOGIN" SASL authentication mechansim transmits the username and password in
+	// plaintext over the internet connection, we only allow this mechanism over a TLS secured
+	// connection.
+	//
+	// https://msopenspecs.azureedge.net/files/MS-XLOGIN/%5bMS-XLOGIN%5d.pdf
+	//
+	// https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00
 	SMTPAuthLogin SMTPAuthType = "LOGIN"
 
 	// SMTPAuthNoAuth is equivalent to performing no authentication at all. It is a convenience
 	// option and should not be used. Instead, for mail servers that do no support/require
-	// authentication, the Client should not be used with the WithSMTPAuth option
+	// authentication, the Client should not be passed the WithSMTPAuth option at all.
 	SMTPAuthNoAuth SMTPAuthType = ""
 
-	// SMTPAuthPlain is the "PLAIN" authentication mechanism as described in RFC 4616
+	// SMTPAuthPlain is the "PLAIN" authentication mechanism as described in RFC 4616.
+	//
+	// Since the "PLAIN" SASL authentication mechansim transmits the username and password in
+	// plaintext over the internet connection, we only allow this mechanism over a TLS secured
+	// connection.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc4616/
 	SMTPAuthPlain SMTPAuthType = "PLAIN"
 
 	// SMTPAuthXOAUTH2 is the "XOAUTH2" SASL authentication mechanism.
 	// https://developers.google.com/gmail/imap/xoauth2-protocol
 	SMTPAuthXOAUTH2 SMTPAuthType = "XOAUTH2"
+
+	// SMTPAuthSCRAMSHA1 is the "SCRAM-SHA-1" SASL authentication mechanism as described in RFC 5802.
+	//
+	// SCRAM-SHA-1 is still considered secure for certain applications, particularly when used as part
+	// of a challenge-response authentication mechanism (as we use it). However, it is generally
+	// recommended to prefer stronger alternatives like SCRAM-SHA-256(-PLUS), as SHA-1 has known
+	// vulnerabilities in other contexts, although it remains effective in HMAC constructions.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc5802
+	SMTPAuthSCRAMSHA1 SMTPAuthType = "SCRAM-SHA-1"
+
+	// SMTPAuthSCRAMSHA1PLUS is the "SCRAM-SHA-1-PLUS" SASL authentication mechanism as described in RFC 5802.
+	//
+	// SCRAM-SHA-X-PLUS authentication require TLS channel bindings to protect against MitM attacks and
+	// to guarantee that the integrity of the transport layer is preserved throughout the authentication
+	// process. Therefore we only allow this mechansim over a TLS secured connection.
+	//
+	// SCRAM-SHA-1-PLUS is still considered secure for certain applications, particularly when used as part
+	// of a challenge-response authentication mechanism (as we use it). However, it is generally
+	// recommended to prefer stronger alternatives like SCRAM-SHA-256(-PLUS), as SHA-1 has known
+	// vulnerabilities in other contexts, although it remains effective in HMAC constructions.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc5802
+	SMTPAuthSCRAMSHA1PLUS SMTPAuthType = "SCRAM-SHA-1-PLUS"
+
+	// SMTPAuthSCRAMSHA256 is the "SCRAM-SHA-256" SASL authentication mechanism as described in RFC 7677.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc7677
+	SMTPAuthSCRAMSHA256 SMTPAuthType = "SCRAM-SHA-256"
+
+	// SMTPAuthSCRAMSHA256PLUS is the "SCRAM-SHA-256-PLUS" SASL authentication mechanism as described in RFC 7677.
+	//
+	// SCRAM-SHA-X-PLUS authentication require TLS channel bindings to protect against MitM attacks and
+	// to guarantee that the integrity of the transport layer is preserved throughout the authentication
+	// process. Therefore we only allow this mechansim over a TLS secured connection.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc7677
+	SMTPAuthSCRAMSHA256PLUS SMTPAuthType = "SCRAM-SHA-256-PLUS"
 )
 
 // SMTP Auth related static errors
 var (
-	// ErrPlainAuthNotSupported should be used if the target server does not support the "PLAIN" schema
+	// ErrPlainAuthNotSupported is returned when the server does not support the "PLAIN" SMTP
+	// authentication type.
 	ErrPlainAuthNotSupported = errors.New("server does not support SMTP AUTH type: PLAIN")
 
-	// ErrLoginAuthNotSupported should be used if the target server does not support the "LOGIN" schema
+	// ErrLoginAuthNotSupported is returned when the server does not support the "LOGIN" SMTP
+	// authentication type.
 	ErrLoginAuthNotSupported = errors.New("server does not support SMTP AUTH type: LOGIN")
 
-	// ErrCramMD5AuthNotSupported should be used if the target server does not support the "CRAM-MD5" schema
+	// ErrCramMD5AuthNotSupported is returned when the server does not support the "CRAM-MD5" SMTP
+	// authentication type.
 	ErrCramMD5AuthNotSupported = errors.New("server does not support SMTP AUTH type: CRAM-MD5")
 
-	// ErrXOauth2AuthNotSupported should be used if the target server does not support the "XOAUTH2" schema
+	// ErrXOauth2AuthNotSupported is returned when the server does not support the "XOAUTH2" schema.
 	ErrXOauth2AuthNotSupported = errors.New("server does not support SMTP AUTH type: XOAUTH2")
+
+	// ErrSCRAMSHA1AuthNotSupported is returned when the server does not support the "SCRAM-SHA-1" SMTP
+	// authentication type.
+	ErrSCRAMSHA1AuthNotSupported = errors.New("server does not support SMTP AUTH type: SCRAM-SHA-1")
+
+	// ErrSCRAMSHA1PLUSAuthNotSupported is returned when the server does not support the "SCRAM-SHA-1-PLUS" SMTP
+	// authentication type.
+	ErrSCRAMSHA1PLUSAuthNotSupported = errors.New("server does not support SMTP AUTH type: SCRAM-SHA-1-PLUS")
+
+	// ErrSCRAMSHA256AuthNotSupported is returned when the server does not support the "SCRAM-SHA-256" SMTP
+	// authentication type.
+	ErrSCRAMSHA256AuthNotSupported = errors.New("server does not support SMTP AUTH type: SCRAM-SHA-256")
+
+	// ErrSCRAMSHA256PLUSAuthNotSupported is returned when the server does not support the "SCRAM-SHA-256-PLUS" SMTP
+	// authentication type.
+	ErrSCRAMSHA256PLUSAuthNotSupported = errors.New("server does not support SMTP AUTH type: SCRAM-SHA-256-PLUS")
 )

b64linebreaker.go

@@ -9,21 +9,39 @@ import (
 	"io"
 )
 
-// ErrNoOutWriter is an error message that should be used if a Base64LineBreaker has no out io.Writer set
+// newlineBytes is a byte slice representation of the SingleNewLine constant used for line breaking
+// in encoding processes.
+var newlineBytes = []byte(SingleNewLine)
+
+// ErrNoOutWriter is the error message returned when no io.Writer is set for Base64LineBreaker.
 const ErrNoOutWriter = "no io.Writer set for Base64LineBreaker"
 
-// Base64LineBreaker is a io.WriteCloser that writes Base64 encoded data streams
+// Base64LineBreaker handles base64 encoding with the insertion of new lines after a certain number
-// with line breaks at a given line length
+// of characters.
+//
+// This struct is used to manage base64 encoding while ensuring that new lines are inserted after
+// reaching a specific line length. It satisfies the io.WriteCloser interface.
+//
+// References:
+//   - https://datatracker.ietf.org/doc/html/rfc2045 (Base64 and line length limitations)
 type Base64LineBreaker struct {
 	line [MaxBodyLength]byte
 	used int
 	out  io.Writer
 }
 
-var newlineBytes = []byte(SingleNewLine)
+// Write writes data to the Base64LineBreaker, ensuring lines do not exceed MaxBodyLength.
-
+//
-// Write writes the data stream and inserts a SingleNewLine when the maximum
+// This method writes the provided data to the Base64LineBreaker. It ensures that the written
-// line length is reached
+// lines do not exceed the MaxBodyLength. If the data exceeds the limit, it handles the
+// continuation by splitting the data and writing new lines as necessary.
+//
+// Parameters:
+//   - data: A byte slice containing the data to be written.
+//
+// Returns:
+//   - numBytes: The number of bytes written.
+//   - err: An error if one occurred during the write operation.
 func (l *Base64LineBreaker) Write(data []byte) (numBytes int, err error) {
 	if l.out == nil {
 		err = errors.New(ErrNoOutWriter)
@@ -55,8 +73,14 @@ func (l *Base64LineBreaker) Write(data []byte) (numBytes int, err error) {
 	return l.Write(data[excess:])
 }
 
-// Close closes the Base64LineBreaker and writes any access data that is still
+// Close finalizes the Base64LineBreaker, writing any remaining buffered data and appending a newline.
-// unwritten in memory
+//
+// This method ensures that any remaining data in the buffer is written to the output and appends
+// a newline. It is used to finalize the Base64LineBreaker and should be called when no more data
+// is expected to be written.
+//
+// Returns:
+//   - err: An error if one occurred during the final write operation.
 func (l *Base64LineBreaker) Close() (err error) {
 	if l.used > 0 {
 		_, err = l.out.Write(l.line[0:l.used])

client_119.go

@@ -7,111 +7,39 @@
 
 package mail
 
-import "strings"
+import "errors"
 
-// Send sends out the mail message
+// Send attempts to send one or more Msg using the Client connection to the SMTP server.
+// If the Client has no active connection to the server, Send will fail with an error. For each
+// of the provided Msg, it will associate a SendError with the Msg in case of a transmission
+// or delivery error.
+//
+// This method first checks for an active connection to the SMTP server. If the connection is
+// not valid, it returns a SendError. It then iterates over the provided messages, attempting
+// to send each one. If an error occurs during sending, the method records the error and
+// associates it with the corresponding Msg. If multiple errors are encountered, it aggregates
+// them into a single SendError to be returned.
+//
+// Parameters:
+//   - messages: A variadic list of pointers to Msg objects to be sent.
+//
+// Returns:
+//   - An error that represents the sending result, which may include multiple SendErrors if
+//     any occurred; otherwise, returns nil.
 func (c *Client) Send(messages ...*Msg) error {
-	if cerr := c.checkConn(); cerr != nil {
+	if err := c.checkConn(); err != nil {
-		return &SendError{Reason: ErrConnCheck, errlist: []error{cerr}, isTemp: isTempError(cerr)}
+		return &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 	}
 	var errs []*SendError
-	for _, message := range messages {
+	for id, message := range messages {
-		message.sendError = nil
+		if sendErr := c.sendSingleMsg(message); sendErr != nil {
-		if message.encoding == NoEncoding {
+			messages[id].sendError = sendErr
-			if ok, _ := c.smtpClient.Extension("8BITMIME"); !ok {
-				sendErr := &SendError{Reason: ErrNoUnencoded, isTemp: false}
-				message.sendError = sendErr
-				errs = append(errs, sendErr)
-				continue
-			}
-		}
-		from, err := message.GetSender(false)
-		if err != nil {
-			sendErr := &SendError{Reason: ErrGetSender, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		rcpts, err := message.GetRecipients()
-		if err != nil {
-			sendErr := &SendError{Reason: ErrGetRcpts, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
 
-		if c.dsn {
+			var msgSendErr *SendError
-			if c.dsnmrtype != "" {
+			if errors.As(sendErr, &msgSendErr) {
-				c.smtpClient.SetDSNMailReturnOption(string(c.dsnmrtype))
+				errs = append(errs, msgSendErr)
 			}
 		}
-		if err = c.smtpClient.Mail(from); err != nil {
-			sendErr := &SendError{Reason: ErrSMTPMailFrom, errlist: []error{err}, isTemp: isTempError(err)}
-			if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
-				sendErr.errlist = append(sendErr.errlist, resetSendErr)
-			}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		failed := false
-		rcptSendErr := &SendError{}
-		rcptSendErr.errlist = make([]error, 0)
-		rcptSendErr.rcpt = make([]string, 0)
-		rcptNotifyOpt := strings.Join(c.dsnrntype, ",")
-		c.smtpClient.SetDSNRcptNotifyOption(rcptNotifyOpt)
-		for _, rcpt := range rcpts {
-			if err = c.smtpClient.Rcpt(rcpt); err != nil {
-				rcptSendErr.Reason = ErrSMTPRcptTo
-				rcptSendErr.errlist = append(rcptSendErr.errlist, err)
-				rcptSendErr.rcpt = append(rcptSendErr.rcpt, rcpt)
-				rcptSendErr.isTemp = isTempError(err)
-				failed = true
-			}
-		}
-		if failed {
-			if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
-				rcptSendErr.errlist = append(rcptSendErr.errlist, err)
-			}
-			message.sendError = rcptSendErr
-			errs = append(errs, rcptSendErr)
-			continue
-		}
-		writer, err := c.smtpClient.Data()
-		if err != nil {
-			sendErr := &SendError{Reason: ErrSMTPData, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		_, err = message.WriteTo(writer)
-		if err != nil {
-			sendErr := &SendError{Reason: ErrWriteContent, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		message.isDelivered = true
-
-		if err = writer.Close(); err != nil {
-			sendErr := &SendError{Reason: ErrSMTPDataClose, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-
-		if err = c.Reset(); err != nil {
-			sendErr := &SendError{Reason: ErrSMTPReset, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		if err = c.checkConn(); err != nil {
-			sendErr := &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
 	}
 
 	if len(errs) > 0 {

client_120.go

@@ -9,101 +9,38 @@ package mail
 
 import (
 	"errors"
-	"strings"
 )
 
-// Send sends out the mail message
+// Send attempts to send one or more Msg using the Client connection to the SMTP server.
+// If the Client has no active connection to the server, Send will fail with an error. For each
+// of the provided Msg, it will associate a SendError with the Msg in case of a transmission
+// or delivery error.
+//
+// This method first checks for an active connection to the SMTP server. If the connection is
+// not valid, it returns an error wrapped in a SendError. It then iterates over the provided
+// messages, attempting to send each one. If an error occurs during sending, the method records
+// the error and associates it with the corresponding Msg.
+//
+// Parameters:
+//   - messages: A variadic list of pointers to Msg objects to be sent.
+//
+// Returns:
+//   - An error that aggregates any SendErrors encountered during the sending process; otherwise, returns nil.
 func (c *Client) Send(messages ...*Msg) (returnErr error) {
 	if err := c.checkConn(); err != nil {
 		returnErr = &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 		return
 	}
-	for _, message := range messages {
-		message.sendError = nil
-		if message.encoding == NoEncoding {
-			if ok, _ := c.smtpClient.Extension("8BITMIME"); !ok {
-				message.sendError = &SendError{Reason: ErrNoUnencoded, isTemp: false}
-				returnErr = errors.Join(returnErr, message.sendError)
-				continue
-			}
-		}
-		from, err := message.GetSender(false)
-		if err != nil {
-			message.sendError = &SendError{Reason: ErrGetSender, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		rcpts, err := message.GetRecipients()
-		if err != nil {
-			message.sendError = &SendError{Reason: ErrGetRcpts, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
 
-		if c.dsn {
+	var errs []error
-			if c.dsnmrtype != "" {
+	defer func() {
-				c.smtpClient.SetDSNMailReturnOption(string(c.dsnmrtype))
+		returnErr = errors.Join(errs...)
-			}
+	}()
-		}
-		if err = c.smtpClient.Mail(from); err != nil {
-			message.sendError = &SendError{Reason: ErrSMTPMailFrom, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
-				returnErr = errors.Join(returnErr, resetSendErr)
-			}
-			continue
-		}
-		failed := false
-		rcptSendErr := &SendError{}
-		rcptSendErr.errlist = make([]error, 0)
-		rcptSendErr.rcpt = make([]string, 0)
-		rcptNotifyOpt := strings.Join(c.dsnrntype, ",")
-		c.smtpClient.SetDSNRcptNotifyOption(rcptNotifyOpt)
-		for _, rcpt := range rcpts {
-			if err = c.smtpClient.Rcpt(rcpt); err != nil {
-				rcptSendErr.Reason = ErrSMTPRcptTo
-				rcptSendErr.errlist = append(rcptSendErr.errlist, err)
-				rcptSendErr.rcpt = append(rcptSendErr.rcpt, rcpt)
-				rcptSendErr.isTemp = isTempError(err)
-				failed = true
-			}
-		}
-		if failed {
-			if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
-				returnErr = errors.Join(returnErr, resetSendErr)
-			}
-			message.sendError = rcptSendErr
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		writer, err := c.smtpClient.Data()
-		if err != nil {
-			message.sendError = &SendError{Reason: ErrSMTPData, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		_, err = message.WriteTo(writer)
-		if err != nil {
-			message.sendError = &SendError{Reason: ErrWriteContent, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		message.isDelivered = true
 
-		if err = writer.Close(); err != nil {
+	for id, message := range messages {
-			message.sendError = &SendError{Reason: ErrSMTPDataClose, errlist: []error{err}, isTemp: isTempError(err)}
+		if sendErr := c.sendSingleMsg(message); sendErr != nil {
-			returnErr = errors.Join(returnErr, message.sendError)
+			messages[id].sendError = sendErr
-			continue
+			errs = append(errs, sendErr)
-		}
-
-		if err = c.Reset(); err != nil {
-			message.sendError = &SendError{Reason: ErrSMTPReset, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		if err = c.checkConn(); err != nil {
-			message.sendError = &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
 		}
 	}
 

doc.go

@@ -2,8 +2,13 @@
 //
 // SPDX-License-Identifier: MIT
 
-// Package mail provides a simple and easy way to send mails with Go
+// Package mail provides an easy to use interface for formating and sending mails. go-mail follows idiomatic Go style
+// and best practice. It has a small dependency footprint by mainly relying on the Go Standard Library and the Go
+// extended packages. It combines a lot of functionality from the standard library to give easy and convenient access
+// to mail and SMTP related tasks. It works like a programatic email client and provides lots of methods and
+// functionalities you would consider standard in a MUA.
 package mail
 
-// VERSION is used in the default user agent string
+// VERSION indicates the current version of the package. It is also attached to the default user
-const VERSION = "0.4.4"
+// agent string.
+const VERSION = "0.5.0"

eml.go

@@ -18,14 +18,35 @@ import (
 	"strings"
 )
 
-// EMLToMsgFromString will parse a given EML string and returns a pre-filled Msg pointer
+// EMLToMsgFromString parses a given EML string and returns a pre-filled Msg pointer.
+//
+// This function takes an EML formatted string, converts it into a bytes buffer, and then
+// calls EMLToMsgFromReader to parse the buffer and create a Msg object. This provides a
+// convenient way to convert EML strings directly into Msg objects.
+//
+// Parameters:
+//   - emlString: A string containing the EML formatted message.
+//
+// Returns:
+//   - A pointer to the Msg object populated with the parsed data, and an error if parsing
+//     fails.
 func EMLToMsgFromString(emlString string) (*Msg, error) {
 	eb := bytes.NewBufferString(emlString)
 	return EMLToMsgFromReader(eb)
 }
 
-// EMLToMsgFromReader will parse a reader that holds EML content and returns a pre-filled
+// EMLToMsgFromReader parses a reader that holds EML content and returns a pre-filled Msg pointer.
-// Msg pointer
+//
+// This function reads EML content from the provided io.Reader and populates a Msg object
+// with the parsed data. It initializes the Msg and extracts headers and body parts from
+// the EML content. Any errors encountered during parsing are returned.
+//
+// Parameters:
+//   - reader: An io.Reader containing the EML formatted message.
+//
+// Returns:
+//   - A pointer to the Msg object populated with the parsed data, and an error if parsing
+//     fails.
 func EMLToMsgFromReader(reader io.Reader) (*Msg, error) {
 	msg := &Msg{
 		addrHeader:    make(map[AddrHeader][]*netmail.Address),
@@ -46,8 +67,19 @@ func EMLToMsgFromReader(reader io.Reader) (*Msg, error) {
 	return msg, nil
 }
 
-// EMLToMsgFromFile will open and parse a .eml file at a provided file path and returns a
+// EMLToMsgFromFile opens and parses a .eml file at a provided file path and returns a
-// pre-filled Msg pointer
+// pre-filled Msg pointer.
+//
+// This function attempts to read and parse an EML file located at the specified file path.
+// It initializes a Msg object and populates it with the parsed headers and body. Any errors
+// encountered during the file operations or parsing are returned.
+//
+// Parameters:
+//   - filePath: The path to the .eml file to be parsed.
+//
+// Returns:
+//   - A pointer to the Msg object populated with the parsed data, and an error if parsing
+//     fails.
 func EMLToMsgFromFile(filePath string) (*Msg, error) {
 	msg := &Msg{
 		addrHeader:    make(map[AddrHeader][]*netmail.Address),
@@ -68,7 +100,19 @@ func EMLToMsgFromFile(filePath string) (*Msg, error) {
 	return msg, nil
 }
 
-// parseEML parses the EML's headers and body and inserts the parsed values into the Msg
+// parseEML parses the EML's headers and body and inserts the parsed values into the Msg.
+//
+// This function extracts relevant header fields and body content from the parsed EML message
+// and stores them in the provided Msg object. It handles various header types and body
+// parts, ensuring that the Msg is correctly populated with all necessary information.
+//
+// Parameters:
+//   - parsedMsg: A pointer to the netmail.Message containing the parsed EML data.
+//   - bodybuf: A bytes.Buffer containing the body content of the EML message.
+//   - msg: A pointer to the Msg object to be populated with the parsed data.
+//
+// Returns:
+//   - An error if any issues occur during the parsing process; otherwise, returns nil.
 func parseEML(parsedMsg *netmail.Message, bodybuf *bytes.Buffer, msg *Msg) error {
 	if err := parseEMLHeaders(&parsedMsg.Header, msg); err != nil {
 		return fmt.Errorf("failed to parse EML headers: %w", err)
@@ -79,7 +123,18 @@ func parseEML(parsedMsg *netmail.Message, bodybuf *bytes.Buffer, msg *Msg) error
 	return nil
 }
 
-// readEML opens an EML file and uses net/mail to parse the header and body
+// readEML opens an EML file and uses net/mail to parse the header and body.
+//
+// This function opens the specified EML file for reading and utilizes the net/mail package
+// to parse the message's headers and body. It returns the parsed message and a buffer
+// containing the body content, along with any errors encountered during the process.
+//
+// Parameters:
+//   - filePath: The path to the EML file to be opened and parsed.
+//
+// Returns:
+//   - A pointer to the parsed netmail.Message, a bytes.Buffer containing the body, and an
+//     error if any issues occur during file operations or parsing.
 func readEML(filePath string) (*netmail.Message, *bytes.Buffer, error) {
 	fileHandle, err := os.Open(filePath)
 	if err != nil {
@@ -91,7 +146,19 @@ func readEML(filePath string) (*netmail.Message, *bytes.Buffer, error) {
 	return readEMLFromReader(fileHandle)
 }
 
-// readEMLFromReader uses net/mail to parse the header and body from a given io.Reader
+// readEMLFromReader uses net/mail to parse the header and body from a given io.Reader.
+//
+// This function reads the EML content from the provided io.Reader and uses the net/mail
+// package to parse the message's headers and body. It returns the parsed netmail.Message
+// along with a bytes.Buffer containing the body content. Any errors encountered during
+// the parsing process are returned.
+//
+// Parameters:
+//   - reader: An io.Reader containing the EML formatted message.
+//
+// Returns:
+//   - A pointer to the parsed netmail.Message, a bytes.Buffer containing the body, and an
+//     error if any issues occur during parsing.
 func readEMLFromReader(reader io.Reader) (*netmail.Message, *bytes.Buffer, error) {
 	parsedMsg, err := netmail.ReadMessage(reader)
 	if err != nil {
@@ -106,8 +173,18 @@ func readEMLFromReader(reader io.Reader) (*netmail.Message, *bytes.Buffer, error
 	return parsedMsg, &buf, nil
 }
 
-// parseEMLHeaders will check the EML headers for the most common headers and set the
+// parseEMLHeaders parses the EML's headers and populates the Msg with relevant information.
-// according settings in the Msg
+//
+// This function checks the EML headers for common headers and sets the corresponding fields
+// in the Msg object. It extracts address headers, content types, and other relevant data
+// for further processing.
+//
+// Parameters:
+//   - mailHeader: A pointer to the netmail.Header containing the EML headers.
+//   - msg: A pointer to the Msg object to be populated with parsed header information.
+//
+// Returns:
+//   - An error if parsing the headers fails; otherwise, returns nil.
 func parseEMLHeaders(mailHeader *netmail.Header, msg *Msg) error {
 	commonHeaders := []Header{
 		HeaderContentType, HeaderImportance, HeaderInReplyTo, HeaderListUnsubscribe,
@@ -175,7 +252,19 @@ func parseEMLHeaders(mailHeader *netmail.Header, msg *Msg) error {
 	return nil
 }
 
-// parseEMLBodyParts parses the body of a EML based on the different content types and encodings
+// parseEMLBodyParts parses the body of an EML based on the different content types and encodings.
+//
+// This function examines the content type of the parsed EML message and processes the body
+// parts accordingly. It handles both plain text and multipart types, ensuring that the
+// Msg object is populated with the appropriate body content.
+//
+// Parameters:
+//   - parsedMsg: A pointer to the netmail.Message containing the parsed EML data.
+//   - bodybuf: A bytes.Buffer containing the body content of the EML message.
+//   - msg: A pointer to the Msg object to be populated with the parsed body content.
+//
+// Returns:
+//   - An error if any issues occur during the body parsing process; otherwise, returns nil.
 func parseEMLBodyParts(parsedMsg *netmail.Message, bodybuf *bytes.Buffer, msg *Msg) error {
 	// Extract the transfer encoding of the body
 	mediatype, params, err := mime.ParseMediaType(parsedMsg.Header.Get(HeaderContentType.String()))
@@ -212,10 +301,24 @@ func parseEMLBodyParts(parsedMsg *netmail.Message, bodybuf *bytes.Buffer, msg *M
 	return nil
 }
 
-// parseEMLBodyPlain parses the mail body of plain type mails
+// parseEMLBodyPlain parses the mail body of plain type messages.
+//
+// This function handles the parsing of plain text messages based on their encoding. It
+// identifies the content transfer encoding and decodes the body content accordingly,
+// storing the result in the provided Msg object.
+//
+// Parameters:
+//   - mediatype: The media type of the message (e.g., text/plain).
+//   - parsedMsg: A pointer to the netmail.Message containing the parsed EML data.
+//   - bodybuf: A bytes.Buffer containing the body content of the EML message.
+//   - msg: A pointer to the Msg object to be populated with the parsed body content.
+//
+// Returns:
+//   - An error if any issues occur during the parsing of the plain body; otherwise, returns nil.
 func parseEMLBodyPlain(mediatype string, parsedMsg *netmail.Message, bodybuf *bytes.Buffer, msg *Msg) error {
 	contentTransferEnc := parsedMsg.Header.Get(HeaderContentTransferEnc.String())
-	// According to RFC2045, if no Content-Transfer-Encoding is set, we can imply 7bit US-ASCII encoding
+	// If no Content-Transfer-Encoding is set, we can imply 7bit US-ASCII encoding
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-6.1
 	if contentTransferEnc == "" || strings.EqualFold(contentTransferEnc, EncodingUSASCII.String()) {
 		msg.SetEncoding(EncodingUSASCII)
 		msg.SetBodyString(ContentType(mediatype), bodybuf.String())
@@ -249,7 +352,20 @@ func parseEMLBodyPlain(mediatype string, parsedMsg *netmail.Message, bodybuf *by
 	return fmt.Errorf("unsupported Content-Transfer-Encoding")
 }
 
-// parseEMLMultipart parses a multipart body part of a EML
+// parseEMLMultipart parses a multipart body part of an EML message.
+//
+// This function handles the parsing of multipart messages, extracting the individual parts
+// and determining their content types. It processes each part according to its content type
+// and ensures that all relevant data is stored in the Msg object.
+//
+// Parameters:
+//   - params: A map containing the parameters from the multipart content type.
+//   - bodybuf: A bytes.Buffer containing the body content of the EML message.
+//   - msg: A pointer to the Msg object to be populated with the parsed body parts.
+//
+// Returns:
+//   - An error if any issues occur during the parsing of the multipart body; otherwise,
+//     returns nil.
 func parseEMLMultipart(params map[string]string, bodybuf *bytes.Buffer, msg *Msg) error {
 	boundary, ok := params["boundary"]
 	if !ok {
@@ -349,7 +465,15 @@ ReadNextPart:
 	return nil
 }
 
-// parseEMLEncoding parses and determines the encoding of the message
+// parseEMLEncoding parses and determines the encoding of the message.
+//
+// This function extracts the content transfer encoding from the EML headers and sets the
+// corresponding encoding in the Msg object. It ensures that the correct encoding is used
+// for further processing of the message content.
+//
+// Parameters:
+//   - mailHeader: A pointer to the netmail.Header containing the EML headers.
+//   - msg: A pointer to the Msg object to be updated with the encoding information.
 func parseEMLEncoding(mailHeader *netmail.Header, msg *Msg) {
 	if value := mailHeader.Get(HeaderContentTransferEnc.String()); value != "" {
 		switch {
@@ -363,7 +487,15 @@ func parseEMLEncoding(mailHeader *netmail.Header, msg *Msg) {
 	}
 }
 
-// parseEMLContentTypeCharset parses and determines the charset and content type of the message
+// parseEMLContentTypeCharset parses and determines the charset and content type of the message.
+//
+// This function extracts the content type and charset from the EML headers, setting them
+// appropriately in the Msg object. It ensures that the Msg object is configured with the
+// correct content type for further processing.
+//
+// Parameters:
+//   - mailHeader: A pointer to the netmail.Header containing the EML headers.
+//   - msg: A pointer to the Msg object to be updated with content type and charset information.
 func parseEMLContentTypeCharset(mailHeader *netmail.Header, msg *Msg) {
 	if value := mailHeader.Get(HeaderContentType.String()); value != "" {
 		contentType, optional := parseMultiPartHeader(value)
@@ -377,7 +509,18 @@ func parseEMLContentTypeCharset(mailHeader *netmail.Header, msg *Msg) {
 	}
 }
 
-// handleEMLMultiPartBase64Encoding sets the content body of a base64 encoded Part
+// handleEMLMultiPartBase64Encoding sets the content body of a base64 encoded Part.
+//
+// This function decodes the base64 encoded content of a multipart part and stores the
+// resulting content in the provided Part object. It handles any errors that occur during
+// the decoding process.
+//
+// Parameters:
+//   - multiPartData: A byte slice containing the base64 encoded data.
+//   - part: A pointer to the Part object where the decoded content will be stored.
+//
+// Returns:
+//   - An error if the base64 decoding fails; otherwise, returns nil.
 func handleEMLMultiPartBase64Encoding(multiPartData []byte, part *Part) error {
 	part.SetEncoding(EncodingB64)
 	content, err := base64.StdEncoding.DecodeString(string(multiPartData))
@@ -388,8 +531,17 @@ func handleEMLMultiPartBase64Encoding(multiPartData []byte, part *Part) error {
 	return nil
 }
 
-// parseMultiPartHeader parses a multipart header and returns the value and optional parts as
+// parseMultiPartHeader parses a multipart header and returns the value and optional parts as a map.
-// separate map
+//
+// This function splits a multipart header into its main value and any optional parameters,
+// returning them separately. It helps in processing multipart messages by extracting
+// relevant information from headers.
+//
+// Parameters:
+//   - multiPartHeader: A string representing the multipart header to be parsed.
+//
+// Returns:
+//   - The main header value as a string and a map of optional parameters.
 func parseMultiPartHeader(multiPartHeader string) (header string, optional map[string]string) {
 	optional = make(map[string]string)
 	headerSplit := strings.SplitN(multiPartHeader, ";", 2)
@@ -404,7 +556,20 @@ func parseMultiPartHeader(multiPartHeader string) (header string, optional map[s
 	return
 }
 
-// parseEMLAttachmentEmbed parses a multipart that is an attachment or embed
+// parseEMLAttachmentEmbed parses a multipart that is an attachment or embed.
+//
+// This function handles the parsing of multipart sections that are marked as attachments or
+// embedded content. It processes the content disposition and sets the appropriate fields in
+// the Msg object based on the parsed data.
+//
+// Parameters:
+//   - contentDisposition: A slice of strings containing the content disposition header.
+//   - multiPart: A pointer to the multipart.Part to be parsed.
+//   - msg: A pointer to the Msg object to be populated with the attachment or embed data.
+//
+// Returns:
+//   - An error if any issues occur during the parsing of attachments or embeds; otherwise,
+//     returns nil.
 func parseEMLAttachmentEmbed(contentDisposition []string, multiPart *multipart.Part, msg *Msg) error {
 	cdType, optional := parseMultiPartHeader(contentDisposition[0])
 	filename := "generic.attachment"

encoding.go

@@ -4,173 +4,226 @@
 
 package mail
 
-// Charset represents a character set for the encoding
+// Charset is a type wrapper for a string representing different character encodings.
 type Charset string
 
-// ContentType represents a content type for the Msg
+// ContentType is a type wrapper for a string and represents the MIME type of the content being handled.
 type ContentType string
 
-// Encoding represents a MIME encoding scheme like quoted-printable or Base64.
+// Encoding is a type wrapper for a string and represents the type of encoding used for email messages
+// and/or parts.
 type Encoding string
 
-// MIMEVersion represents the MIME version for the mail
+// MIMEVersion is a type wrapper for a string nad represents the MIME version used in email messages.
 type MIMEVersion string
 
-// MIMEType represents the MIME type for the mail
+// MIMEType is a type wrapper for a string and represents the MIME type for the Msg content or parts.
 type MIMEType string
 
-// List of supported encodings
 const (
 	// EncodingB64 represents the Base64 encoding as specified in RFC 2045.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-6.8
 	EncodingB64 Encoding = "base64"
 
 	// EncodingQP represents the "quoted-printable" encoding as specified in RFC 2045.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-6.7
 	EncodingQP Encoding = "quoted-printable"
 
 	// EncodingUSASCII represents encoding with only US-ASCII characters (aka 7Bit)
+	//
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-2.7
 	EncodingUSASCII Encoding = "7bit"
 
-	// NoEncoding avoids any character encoding (except of the mail headers)
+	// NoEncoding represents 8-bit encoding for email messages as specified in RFC 6152.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-2.8
+	//
+	// https://datatracker.ietf.org/doc/html/rfc6152
 	NoEncoding Encoding = "8bit"
 )
 
-// List of common charsets
 const (
-	// CharsetUTF7 represents the "UTF-7" charset
+	// CharsetUTF7 represents the "UTF-7" charset.
 	CharsetUTF7 Charset = "UTF-7"
 
-	// CharsetUTF8 represents the "UTF-8" charset
+	// CharsetUTF8 represents the "UTF-8" charset.
 	CharsetUTF8 Charset = "UTF-8"
 
-	// CharsetASCII represents the "US-ASCII" charset
+	// CharsetASCII represents the "US-ASCII" charset.
 	CharsetASCII Charset = "US-ASCII"
 
-	// CharsetISO88591 represents the "ISO-8859-1" charset
+	// CharsetISO88591 represents the "ISO-8859-1" charset.
 	CharsetISO88591 Charset = "ISO-8859-1"
 
-	// CharsetISO88592 represents the "ISO-8859-2" charset
+	// CharsetISO88592 represents the "ISO-8859-2" charset.
 	CharsetISO88592 Charset = "ISO-8859-2"
 
-	// CharsetISO88593 represents the "ISO-8859-3" charset
+	// CharsetISO88593 represents the "ISO-8859-3" charset.
 	CharsetISO88593 Charset = "ISO-8859-3"
 
-	// CharsetISO88594 represents the "ISO-8859-4" charset
+	// CharsetISO88594 represents the "ISO-8859-4" charset.
 	CharsetISO88594 Charset = "ISO-8859-4"
 
-	// CharsetISO88595 represents the "ISO-8859-5" charset
+	// CharsetISO88595 represents the "ISO-8859-5" charset.
 	CharsetISO88595 Charset = "ISO-8859-5"
 
-	// CharsetISO88596 represents the "ISO-8859-6" charset
+	// CharsetISO88596 represents the "ISO-8859-6" charset.
 	CharsetISO88596 Charset = "ISO-8859-6"
 
-	// CharsetISO88597 represents the "ISO-8859-7" charset
+	// CharsetISO88597 represents the "ISO-8859-7" charset.
 	CharsetISO88597 Charset = "ISO-8859-7"
 
-	// CharsetISO88599 represents the "ISO-8859-9" charset
+	// CharsetISO88599 represents the "ISO-8859-9" charset.
 	CharsetISO88599 Charset = "ISO-8859-9"
 
-	// CharsetISO885913 represents the "ISO-8859-13" charset
+	// CharsetISO885913 represents the "ISO-8859-13" charset.
 	CharsetISO885913 Charset = "ISO-8859-13"
 
-	// CharsetISO885914 represents the "ISO-8859-14" charset
+	// CharsetISO885914 represents the "ISO-8859-14" charset.
 	CharsetISO885914 Charset = "ISO-8859-14"
 
-	// CharsetISO885915 represents the "ISO-8859-15" charset
+	// CharsetISO885915 represents the "ISO-8859-15" charset.
 	CharsetISO885915 Charset = "ISO-8859-15"
 
-	// CharsetISO885916 represents the "ISO-8859-16" charset
+	// CharsetISO885916 represents the "ISO-8859-16" charset.
 	CharsetISO885916 Charset = "ISO-8859-16"
 
-	// CharsetISO2022JP represents the "ISO-2022-JP" charset
+	// CharsetISO2022JP represents the "ISO-2022-JP" charset.
 	CharsetISO2022JP Charset = "ISO-2022-JP"
 
-	// CharsetISO2022KR represents the "ISO-2022-KR" charset
+	// CharsetISO2022KR represents the "ISO-2022-KR" charset.
 	CharsetISO2022KR Charset = "ISO-2022-KR"
 
-	// CharsetWindows1250 represents the "windows-1250" charset
+	// CharsetWindows1250 represents the "windows-1250" charset.
 	CharsetWindows1250 Charset = "windows-1250"
 
-	// CharsetWindows1251 represents the "windows-1251" charset
+	// CharsetWindows1251 represents the "windows-1251" charset.
 	CharsetWindows1251 Charset = "windows-1251"
 
-	// CharsetWindows1252 represents the "windows-1252" charset
+	// CharsetWindows1252 represents the "windows-1252" charset.
 	CharsetWindows1252 Charset = "windows-1252"
 
-	// CharsetWindows1255 represents the "windows-1255" charset
+	// CharsetWindows1255 represents the "windows-1255" charset.
 	CharsetWindows1255 Charset = "windows-1255"
 
-	// CharsetWindows1256 represents the "windows-1256" charset
+	// CharsetWindows1256 represents the "windows-1256" charset.
 	CharsetWindows1256 Charset = "windows-1256"
 
-	// CharsetKOI8R represents the "KOI8-R" charset
+	// CharsetKOI8R represents the "KOI8-R" charset.
 	CharsetKOI8R Charset = "KOI8-R"
 
-	// CharsetKOI8U represents the "KOI8-U" charset
+	// CharsetKOI8U represents the "KOI8-U" charset.
 	CharsetKOI8U Charset = "KOI8-U"
 
-	// CharsetBig5 represents the "Big5" charset
+	// CharsetBig5 represents the "Big5" charset.
 	CharsetBig5 Charset = "Big5"
 
-	// CharsetGB18030 represents the "GB18030" charset
+	// CharsetGB18030 represents the "GB18030" charset.
 	CharsetGB18030 Charset = "GB18030"
 
-	// CharsetGB2312 represents the "GB2312" charset
+	// CharsetGB2312 represents the "GB2312" charset.
 	CharsetGB2312 Charset = "GB2312"
 
-	// CharsetTIS620 represents the "TIS-620" charset
+	// CharsetTIS620 represents the "TIS-620" charset.
 	CharsetTIS620 Charset = "TIS-620"
 
-	// CharsetEUCKR represents the "EUC-KR" charset
+	// CharsetEUCKR represents the "EUC-KR" charset.
 	CharsetEUCKR Charset = "EUC-KR"
 
-	// CharsetShiftJIS represents the "Shift_JIS" charset
+	// CharsetShiftJIS represents the "Shift_JIS" charset.
 	CharsetShiftJIS Charset = "Shift_JIS"
 
-	// CharsetUnknown represents the "Unknown" charset
+	// CharsetUnknown represents the "Unknown" charset.
 	CharsetUnknown Charset = "Unknown"
 
-	// CharsetGBK represents the "GBK" charset
+	// CharsetGBK represents the "GBK" charset.
 	CharsetGBK Charset = "GBK"
 )
 
-// List of MIME versions
+// MIME10 represents the MIME version "1.0" used in email messages.
-const (
+const MIME10 MIMEVersion = "1.0"
-	// MIME10 is the MIME Version 1.0
-	MIME10 MIMEVersion = "1.0"
-)
 
-// List of common content types
 const (
-	TypeAppOctetStream       ContentType = "application/octet-stream"
+	// TypeAppOctetStream represents the MIME type for arbitrary binary data.
+	TypeAppOctetStream ContentType = "application/octet-stream"
+
+	// TypeMultipartAlternative represents the MIME type for a message body that can contain multiple alternative
+	// formats.
 	TypeMultipartAlternative ContentType = "multipart/alternative"
-	TypeMultipartMixed       ContentType = "multipart/mixed"
+
-	TypeMultipartRelated     ContentType = "multipart/related"
+	// TypeMultipartMixed represents the MIME type for a multipart message containing different parts.
-	TypePGPSignature         ContentType = "application/pgp-signature"
+	TypeMultipartMixed ContentType = "multipart/mixed"
-	TypePGPEncrypted         ContentType = "application/pgp-encrypted"
+
-	TypeTextHTML             ContentType = "text/html"
+	// TypeMultipartRelated represents the MIME type for a multipart message where each part is a related file
-	TypeTextPlain            ContentType = "text/plain"
+	// or resource.
-	typeSMimeSigned          ContentType = `application/pkcs7-signature; name="smime.p7s"`
+	TypeMultipartRelated ContentType = "multipart/related"
+
+	// TypePGPSignature represents the MIME type for PGP signed messages.
+	TypePGPSignature ContentType = "application/pgp-signature"
+
+	// TypePGPEncrypted represents the MIME type for PGP encrypted messages.
+	TypePGPEncrypted ContentType = "application/pgp-encrypted"
+
+	// TypeTextHTML represents the MIME type for HTML text content.
+	TypeTextHTML ContentType = "text/html"
+
+	// TypeTextPlain represents the MIME type for plain text content.
+	TypeTextPlain ContentType = "text/plain"
+  
+  // typeSMimeSigned represents the MIME type for S/MIME singed messages.
+  typeSMimeSigned ContentType = `application/pkcs7-signature; name="smime.p7s"`
 )
 
-// List of MIMETypes
 const (
+	// MIMEAlternative MIMEType represents a MIME multipart/alternative type, used for emails with multiple versions.
 	MIMEAlternative MIMEType = "alternative"
+  // MIMEMixed MIMEType represents a MIME multipart/mixed type used for emails containing different types of content.
 	MIMEMixed       MIMEType = "mixed"
+  // MIMERelated MIMEType represents a MIME multipart/related type, used for emails with related content entities.
 	MIMERelated     MIMEType = "related"
-	MIMESMime       MIMEType = `signed; protocol="application/pkcs7-signature"; micalg=sha256`
+  // MIMESMime MIMEType represents a MIME multipart/signed type, used for siging emails with S/MIME.
+	MIMESMime       MIMEType = `signed; protocol="application/pkcs7-signature"; micalg=sha-256`
 )
 
-// String is a standard method to convert an Charset into a printable format
+// String satisfies the fmt.Stringer interface for the Charset type.
+// It converts a Charset into a printable format.
+//
+// This method returns the string representation of the Charset, allowing it to be easily
+// printed or logged.
+//
+// Returns:
+//   - A string representation of the Charset.
 func (c Charset) String() string {
 	return string(c)
 }
 
-// String is a standard method to convert an ContentType into a printable format
+// String satisfies the fmt.Stringer interface for the ContentType type.
+// It converts a ContentType into a printable format.
+//
+// This method returns the string representation of the ContentType, enabling its use
+// in formatted output such as logging or displaying information to the user.
+//
+// Returns:
+//   - A string representation of the ContentType.
 func (c ContentType) String() string {
 	return string(c)
 }
 
-// String is a standard method to convert an Encoding into a printable format
+// String satisfies the fmt.Stringer interface for the Encoding type.
+// It converts an Encoding into a printable format.
+//
+// This method returns the string representation of the Encoding, which can be used
+// for displaying or logging purposes.
+//
+// Returns:
+//   - A string representation of the Encoding.
 func (e Encoding) String() string {
 	return string(e)
 }
+
+// String is a standard method to convert an MIMEType into a printable format
+func (e MIMEType) String() string {
+	return string(e)
+}

encoding_test.go

@@ -126,3 +126,24 @@ func TestCharset_String(t *testing.T) {
 		})
 	}
 }
+
+// TestContentType_String tests the mime type method of the MIMEType object
+func TestMimeType_String(t *testing.T) {
+	tests := []struct {
+		mt   MIMEType
+		want string
+	}{
+		{MIMEAlternative, "alternative"},
+		{MIMEMixed, "mixed"},
+		{MIMERelated, "related"},
+		{MIMESMime, `signed; protocol="application/pkcs7-signature"; micalg=sha-256`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.mt.String(), func(t *testing.T) {
+			if tt.mt.String() != tt.want {
+				t.Errorf("wrong string for mime type returned. Expected: %s, got: %s",
+					tt.want, tt.mt.String())
+			}
+		})
+	}
+}

file.go

@@ -9,10 +9,15 @@ import (
 	"net/textproto"
 )
 
-// FileOption returns a function that can be used for grouping File options
+// FileOption is a function type used to modify properties of a File
 type FileOption func(*File)
 
-// File is an attachment or embedded file of the Msg
+// File represents a file with properties such as content type, description, encoding, headers, name, and
+// a writer function.
+//
+// This struct can represent either an attachment or an embedded file in a Msg, and it stores relevant
+// metadata such as content type and encoding, as well as a function to write the file's content to an
+// io.Writer.
 type File struct {
 	ContentType ContentType
 	Desc        string
@@ -22,32 +27,68 @@ type File struct {
 	Writer      func(w io.Writer) (int64, error)
 }
 
-// WithFileContentID sets the Content-ID header for the File
+// WithFileContentID sets the "Content-ID" header in the File's MIME headers to the specified ID.
+//
+// This function updates the File's MIME headers by setting the "Content-ID" to the provided string value,
+// allowing the file to be referenced by this ID within the MIME structure.
+//
+// Parameters:
+//   - id: A string representing the content ID to be set in the "Content-ID" header.
+//
+// Returns:
+//   - A FileOption function that updates the File's "Content-ID" header.
 func WithFileContentID(id string) FileOption {
 	return func(f *File) {
 		f.Header.Set(HeaderContentID.String(), id)
 	}
 }
 
-// WithFileName sets the filename of the File
+// WithFileName sets the name of a File to the provided value.
+//
+// This function assigns the specified name to the File, updating its Name field.
+//
+// Parameters:
+//   - name: A string representing the name to be assigned to the File.
+//
+// Returns:
+//   - A FileOption function that sets the File's name.
 func WithFileName(name string) FileOption {
 	return func(f *File) {
 		f.Name = name
 	}
 }
 
-// WithFileDescription sets an optional file description of the File that will be
+// WithFileDescription sets an optional description for the File, which is used in the Content-Description
-// added as Content-Description part
+// header of the MIME output.
+//
+// This function updates the File's description, allowing an additional text description to be added to
+// the MIME headers for the file.
+//
+// Parameters:
+//   - description: A string representing the description to be set in the Content-Description header.
+//
+// Returns:
+//   - A FileOption function that sets the File's description.
 func WithFileDescription(description string) FileOption {
 	return func(f *File) {
 		f.Desc = description
 	}
 }
 
-// WithFileEncoding sets the encoding of the File. By default we should always use
+// WithFileEncoding sets the encoding type for a File.
-// Base64 encoding but there might be exceptions, where this might come handy.
+//
-// Please note that quoted-printable should never be used for attachments/embeds. If this
+// This function allows the specification of an encoding type for the file, typically used for attachments
-// is provided as argument, the function will automatically override back to Base64
+// or embedded files. By default, Base64 encoding should be used, but this function can override the
+// default if needed.
+//
+// Note: Quoted-printable encoding (EncodingQP) must never be used for attachments or embeds. If EncodingQP
+// is passed to this function, it will be ignored and the encoding will remain unchanged.
+//
+// Parameters:
+//   - encoding: The Encoding type to be assigned to the File, unless it's EncodingQP.
+//
+// Returns:
+//   - A FileOption function that sets the File's encoding.
 func WithFileEncoding(encoding Encoding) FileOption {
 	return func(f *File) {
 		if encoding == EncodingQP {
@@ -58,23 +99,45 @@ func WithFileEncoding(encoding Encoding) FileOption {
 }
 
 // WithFileContentType sets the content type of the File.
-// By default go-mail will try to guess the file type and its corresponding
+//
-// content type and fall back to application/octet-stream if the file type
+// By default, the content type is guessed based on the file type, and if no matching type is identified,
-// could not be guessed. In some cases, however, it might be needed to force
+// the default "application/octet-stream" is used. This FileOption allows overriding the guessed content
-// this to a specific type. For such situations this override method can
+// type with a specific one if required.
-// be used
+//
+// Parameters:
+//   - contentType: The ContentType to be assigned to the File.
+//
+// Returns:
+//   - A FileOption function that sets the File's content type.
 func WithFileContentType(contentType ContentType) FileOption {
 	return func(f *File) {
 		f.ContentType = contentType
 	}
 }
 
-// setHeader sets header fields to a File
+// setHeader sets the value of a specified MIME header field for the File.
+//
+// This method updates the MIME headers of the File by assigning the provided value to the specified
+// header field.
+//
+// Parameters:
+//   - header: The Header field to be updated.
+//   - value: A string representing the value to be set for the given header.
 func (f *File) setHeader(header Header, value string) {
 	f.Header.Set(string(header), value)
 }
 
-// getHeader return header fields of a File
+// getHeader retrieves the value of the specified MIME header field.
+//
+// This method returns the value of the given header and a boolean indicating whether the header was found
+// in the File's MIME headers.
+//
+// Parameters:
+//   - header: The Header field whose value is to be retrieved.
+//
+// Returns:
+//   - A string containing the value of the header.
+//   - A boolean indicating whether the header was present (true) or not (false).
 func (f *File) getHeader(header Header) (string, bool) {
 	v := f.Header.Get(string(header))
 	return v, v != ""

go.mod

@@ -6,4 +6,8 @@ module github.com/wneessen/go-mail
 
 go 1.16
 
-require go.mozilla.org/pkcs7 v0.9.0
+require (
+	go.mozilla.org/pkcs7 v0.9.0
+	golang.org/x/crypto v0.28.0
+	golang.org/x/text v0.19.0
+)

go.sum

@@ -1,2 +1,68 @@
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
+golang.org/x/crypto v0.13.0/go.mod h1:y6Z2r+Rw4iayiXXAIxJIDAJ1zMW4yaTpebo8fPOliYc=
+golang.org/x/crypto v0.19.0/go.mod h1:Iy9bg/ha4yyC70EfRS8jz+B6ybOBKMaSxLj6P6oBDfU=
+golang.org/x/crypto v0.23.0/go.mod h1:CKFgDieR+mRhux2Lsu27y0fO304Db0wZe70UKqHu0v8=
+golang.org/x/crypto v0.28.0 h1:GBDwsMXVQi34v5CCYUm2jkJvu4cbtru2U4TN2PSyQnw=
+golang.org/x/crypto v0.28.0/go.mod h1:rmgy+3RHxRZMyY0jjAJShp2zgEdOqj2AO7U0pYmeQ7U=
+golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
+golang.org/x/mod v0.8.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
+golang.org/x/mod v0.12.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
+golang.org/x/mod v0.15.0/go.mod h1:hTbmBsO62+eylJbnUtE2MGJUyE7QWk4xUqPFrRgJ+7c=
+golang.org/x/mod v0.17.0/go.mod h1:hTbmBsO62+eylJbnUtE2MGJUyE7QWk4xUqPFrRgJ+7c=
+golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
+golang.org/x/net v0.0.0-20220722155237-a158d28d115b/go.mod h1:XRhObCWvk6IyKnWLug+ECip1KBveYUHfp+8e9klMJ9c=
+golang.org/x/net v0.6.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
+golang.org/x/net v0.10.0/go.mod h1:0qNGK6F8kojg2nk9dLZ2mShWaEBan6FAoqfSigmmuDg=
+golang.org/x/net v0.15.0/go.mod h1:idbUs1IY1+zTqbi8yxTbhexhEEk5ur9LInksu6HrEpk=
+golang.org/x/net v0.21.0/go.mod h1:bIjVDfnllIU7BJ2DNgfnXvpSvtn8VRwhlsaeUTyUS44=
+golang.org/x/net v0.25.0/go.mod h1:JkAGAh7GEvH74S6FOH42FLoXpXbE/aqXSrIQjXgsiwM=
 go.mozilla.org/pkcs7 v0.9.0 h1:yM4/HS9dYv7ri2biPtxt8ikvB37a980dg69/pKmS+eI=
 go.mozilla.org/pkcs7 v0.9.0/go.mod h1:SNgMg+EgDFwmvSmLRTNKC5fegJjB7v23qTQ0XLGUNHk=
+golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.3.0/go.mod h1:FU7BRWz2tNW+3quACPkgCx/L+uEAv1htQ0V83Z9Rj+Y=
+golang.org/x/sync v0.6.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
+golang.org/x/sync v0.7.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
+golang.org/x/sync v0.8.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.5.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.8.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.12.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.17.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/sys v0.20.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/sys v0.26.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/telemetry v0.0.0-20240228155512-f48c80bd79b2/go.mod h1:TeRTkGYfJXctD9OcfyVLyj2J3IxLnKwHJR8f4D8a3YE=
+golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
+golang.org/x/term v0.0.0-20210927222741-03fcf44c2211/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
+golang.org/x/term v0.5.0/go.mod h1:jMB1sMXY+tzblOD4FWmEbocvup2/aLOaQEp7JmGp78k=
+golang.org/x/term v0.8.0/go.mod h1:xPskH00ivmX89bAKVGSKKtLOWNx2+17Eiy94tnKShWo=
+golang.org/x/term v0.12.0/go.mod h1:owVbMEjm3cBLCHdkQu9b1opXd4ETQWc3BhuQGKgXgvU=
+golang.org/x/term v0.17.0/go.mod h1:lLRBjIVuehSbZlaOtGMbcMncT+aqLLLmKrsjNrUguwk=
+golang.org/x/term v0.20.0/go.mod h1:8UkIAJTvZgivsXaD6/pH6U9ecQzZ45awqEOzuCvwpFY=
+golang.org/x/term v0.25.0/go.mod h1:RPyXicDX+6vLxogjjRxjgD2TKtmAO6NZBsBRfrOLu7M=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
+golang.org/x/text v0.7.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
+golang.org/x/text v0.9.0/go.mod h1:e1OnstbJyHTd6l/uOt8jFFHp6TRDWZR/bV3emEE/zU8=
+golang.org/x/text v0.13.0/go.mod h1:TvPlkZtksWOMsz7fbANvkp4WM8x/WCo/om8BMLbz+aE=
+golang.org/x/text v0.14.0/go.mod h1:18ZOQIKpY8NJVqYksKHtTdi31H5itFRjB5/qKTNYzSU=
+golang.org/x/text v0.15.0/go.mod h1:18ZOQIKpY8NJVqYksKHtTdi31H5itFRjB5/qKTNYzSU=
+golang.org/x/text v0.19.0 h1:kTxAhCbGbxhK0IwgSKiMO5awPoDQ0RpfiVYBfK860YM=
+golang.org/x/text v0.19.0/go.mod h1:BuEKDfySbSR4drPmRPG/7iBdf8hvFMuRexcpahXilzY=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
+golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=
+golang.org/x/tools v0.6.0/go.mod h1:Xwgl3UAJ/d3gWutnCtw505GrjyAbvKui8lOU390QaIU=
+golang.org/x/tools v0.13.0/go.mod h1:HvlwmtVNQAhOuCjW7xxvovg8wbNq7LwfXh/k7wXUl58=
+golang.org/x/tools v0.21.1-0.20240508182429-e35e4ccd0d2d/go.mod h1:aiJjzUbINMkxbQROHiO6hDPo2LHcIPhhQsa9DLh0yGk=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

go.sum.license

@@ -0,0 +1,3 @@
+// SPDX-FileCopyrightText: Copyright (c) 2022-2024 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT

header.go

@@ -4,129 +4,146 @@
 
 package mail
 
-// Header represents a generic mail header field name
+// Header is a type wrapper for a string and represents email header fields in a Msg.
 type Header string
 
-// AddrHeader represents a address related mail Header field name
+// AddrHeader is a type wrapper for a string and represents email address headers fields in a Msg.
 type AddrHeader string
 
-// Importance represents a Importance/Priority value string
+// Importance is a type wrapper for an int and represents the level of importance or priority for a Msg.
 type Importance int
 
-// List of common generic header field names
 const (
-	// HeaderContentDescription is the "Content-Description" header
+	// HeaderContentDescription is the "Content-Description" header.
 	HeaderContentDescription Header = "Content-Description"
 
-	// HeaderContentDisposition is the "Content-Disposition" header
+	// HeaderContentDisposition is the "Content-Disposition" header.
 	HeaderContentDisposition Header = "Content-Disposition"
 
-	// HeaderContentID is the "Content-ID" header
+	// HeaderContentID is the "Content-ID" header.
 	HeaderContentID Header = "Content-ID"
 
-	// HeaderContentLang is the "Content-Language" header
+	// HeaderContentLang is the "Content-Language" header.
 	HeaderContentLang Header = "Content-Language"
 
-	// HeaderContentLocation is the "Content-Location" header (RFC 2110)
+	// HeaderContentLocation is the "Content-Location" header (RFC 2110).
+	// https://datatracker.ietf.org/doc/html/rfc2110#section-4.3
 	HeaderContentLocation Header = "Content-Location"
 
-	// HeaderContentTransferEnc is the "Content-Transfer-Encoding" header
+	// HeaderContentTransferEnc is the "Content-Transfer-Encoding" header.
 	HeaderContentTransferEnc Header = "Content-Transfer-Encoding"
 
-	// HeaderContentType is the "Content-Type" header
+	// HeaderContentType is the "Content-Type" header.
 	HeaderContentType Header = "Content-Type"
 
-	// HeaderDate represents the "Date" field
+	// HeaderDate represents the "Date" field.
-	// See: https://www.rfc-editor.org/rfc/rfc822#section-5.1
+	// https://datatracker.ietf.org/doc/html/rfc822#section-5.1
 	HeaderDate Header = "Date"
 
-	// HeaderDispositionNotificationTo is the MDN header as described in RFC8098
+	// HeaderDispositionNotificationTo is the MDN header as described in RFC 8098.
-	// See: https://www.rfc-editor.org/rfc/rfc8098.html#section-2.1
+	// https://datatracker.ietf.org/doc/html/rfc8098#section-2.1
 	HeaderDispositionNotificationTo Header = "Disposition-Notification-To"
 
-	// HeaderImportance represents the "Importance" field
+	// HeaderImportance represents the "Importance" field.
 	HeaderImportance Header = "Importance"
 
-	// HeaderInReplyTo represents the "In-Reply-To" field
+	// HeaderInReplyTo represents the "In-Reply-To" field.
 	HeaderInReplyTo Header = "In-Reply-To"
 
-	// HeaderListUnsubscribe is the "List-Unsubscribe" header field
+	// HeaderListUnsubscribe is the "List-Unsubscribe" header field.
 	HeaderListUnsubscribe Header = "List-Unsubscribe"
 
-	// HeaderListUnsubscribePost is the "List-Unsubscribe-Post" header field
+	// HeaderListUnsubscribePost is the "List-Unsubscribe-Post" header field.
 	HeaderListUnsubscribePost Header = "List-Unsubscribe-Post"
 
-	// HeaderMessageID represents the "Message-ID" field for message identification
+	// HeaderMessageID represents the "Message-ID" field for message identification.
-	// See: https://www.rfc-editor.org/rfc/rfc1036#section-2.1.5
+	// https://datatracker.ietf.org/doc/html/rfc1036#section-2.1.5
 	HeaderMessageID Header = "Message-ID"
 
-	// HeaderMIMEVersion represents the "MIME-Version" field as per RFC 2045
+	// HeaderMIMEVersion represents the "MIME-Version" field as per RFC 2045.
-	// See: https://datatracker.ietf.org/doc/html/rfc2045#section-4
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-4
 	HeaderMIMEVersion Header = "MIME-Version"
 
-	// HeaderOrganization is the "Organization" header field
+	// HeaderOrganization is the "Organization" header field.
 	HeaderOrganization Header = "Organization"
 
-	// HeaderPrecedence is the "Precedence" header field
+	// HeaderPrecedence is the "Precedence" header field.
 	HeaderPrecedence Header = "Precedence"
 
-	// HeaderPriority represents the "Priority" field
+	// HeaderPriority represents the "Priority" field.
 	HeaderPriority Header = "Priority"
 
-	// HeaderReferences is the "References" header field
+	// HeaderReferences is the "References" header field.
 	HeaderReferences Header = "References"
 
-	// HeaderReplyTo is the "Reply-To" header field
+	// HeaderReplyTo is the "Reply-To" header field.
 	HeaderReplyTo Header = "Reply-To"
 
-	// HeaderSubject is the "Subject" header field
+	// HeaderSubject is the "Subject" header field.
 	HeaderSubject Header = "Subject"
 
-	// HeaderUserAgent is the "User-Agent" header field
+	// HeaderUserAgent is the "User-Agent" header field.
 	HeaderUserAgent Header = "User-Agent"
 
-	// HeaderXAutoResponseSuppress is the "X-Auto-Response-Suppress" header field
+	// HeaderXAutoResponseSuppress is the "X-Auto-Response-Suppress" header field.
 	HeaderXAutoResponseSuppress Header = "X-Auto-Response-Suppress"
 
-	// HeaderXMailer is the "X-Mailer" header field
+	// HeaderXMailer is the "X-Mailer" header field.
 	HeaderXMailer Header = "X-Mailer"
 
-	// HeaderXMSMailPriority is the "X-MSMail-Priority" header field
+	// HeaderXMSMailPriority is the "X-MSMail-Priority" header field.
 	HeaderXMSMailPriority Header = "X-MSMail-Priority"
 
-	// HeaderXPriority is the "X-Priority" header field
+	// HeaderXPriority is the "X-Priority" header field.
 	HeaderXPriority Header = "X-Priority"
 )
 
-// List of common address header field names
 const (
-	// HeaderBcc is the "Blind Carbon Copy" header field
+	// HeaderBcc is the "Blind Carbon Copy" header field.
 	HeaderBcc AddrHeader = "Bcc"
 
-	// HeaderCc is the "Carbon Copy" header field
+	// HeaderCc is the "Carbon Copy" header field.
 	HeaderCc AddrHeader = "Cc"
 
-	// HeaderEnvelopeFrom is the envelope FROM header field
+	// HeaderEnvelopeFrom is the envelope FROM header field.
-	// It's not included in the mail body but only used by the Client for the envelope
+	//
+	// It is generally not included in the mail body but only used by the Client for the communication with the
+	// SMTP server. If the Msg has no "FROM" address set in the mail body, the msgWriter will try to use the
+	// envelope from address, if this has been set for the Msg.
 	HeaderEnvelopeFrom AddrHeader = "EnvelopeFrom"
 
-	// HeaderFrom is the "From" header field
+	// HeaderFrom is the "From" header field.
 	HeaderFrom AddrHeader = "From"
 
-	// HeaderTo is the "Receipient" header field
+	// HeaderTo is the "Receipient" header field.
 	HeaderTo AddrHeader = "To"
 )
 
-// List of Importance values
 const (
+	// ImportanceLow indicates a low level of importance or priority in a Msg.
 	ImportanceLow Importance = iota
+
+	// ImportanceNormal indicates a standard level of importance or priority for a Msg.
 	ImportanceNormal
+
+	// ImportanceHigh indicates a high level of importance or priority in a Msg.
 	ImportanceHigh
+
+	// ImportanceNonUrgent indicates a non-urgent level of importance or priority in a Msg.
 	ImportanceNonUrgent
+
+	// ImportanceUrgent indicates an urgent level of importance or priority in a Msg.
 	ImportanceUrgent
 )
 
-// NumString returns the importance number string based on the Importance
+// NumString returns a numerical string representation of the Importance level.
+//
+// This method maps ImportanceHigh and ImportanceUrgent to "1", while ImportanceNonUrgent and ImportanceLow
+// are mapped to "0". Other values return an empty string.
+//
+// Returns:
+//   - A string representing the numerical value of the Importance level ("1" or "0"), or an empty string
+//     if the Importance level is unrecognized.
 func (i Importance) NumString() string {
 	switch i {
 	case ImportanceNonUrgent:
@@ -142,7 +159,14 @@ func (i Importance) NumString() string {
 	}
 }
 
-// XPrioString returns the X-Priority number string based on the Importance
+// XPrioString returns the X-Priority string representation of the Importance level.
+//
+// This method maps ImportanceHigh and ImportanceUrgent to "1", while ImportanceNonUrgent and ImportanceLow
+// are mapped to "5". Other values return an empty string.
+//
+// Returns:
+//   - A string representing the X-Priority value of the Importance level ("1" or "5"), or an empty string
+//     if the Importance level is unrecognized.
 func (i Importance) XPrioString() string {
 	switch i {
 	case ImportanceNonUrgent:
@@ -158,7 +182,14 @@ func (i Importance) XPrioString() string {
 	}
 }
 
-// String returns the importance string based on the Importance
+// String satisfies the fmt.Stringer interface for the Importance type and returns the string
+// representation of the Importance level.
+//
+// This method provides a human-readable string for each Importance level.
+//
+// Returns:
+//   - A string representing the Importance level ("non-urgent", "low", "high", or "urgent"), or an empty
+//     string if the Importance level is unrecognized.
 func (i Importance) String() string {
 	switch i {
 	case ImportanceNonUrgent:
@@ -174,12 +205,20 @@ func (i Importance) String() string {
 	}
 }
 
-// String returns the header string based on the given Header
+// String satisfies the fmt.Stringer interface for the Header type and returns the string
+// representation of the Header.
+//
+// Returns:
+//   - A string representing the Header.
 func (h Header) String() string {
 	return string(h)
 }
 
-// String returns the address header string based on the given AddrHeader
+// String satisfies the fmt.Stringer interface for the AddrHeader type and returns the string
+// representation of the AddrHeader.
+//
+// Returns:
+//   - A string representing the AddrHeader.
 func (a AddrHeader) String() string {
 	return string(a)
 }

msg_nowin_test.go

@@ -61,3 +61,25 @@ func TestMsg_WriteToSendmail(t *testing.T) {
 		t.Errorf("WriteToSendmail failed: %s", err)
 	}
 }
+
+func TestMsg_WriteToTempFileFailed(t *testing.T) {
+	m := NewMsg()
+	_ = m.From("Toni Tester <tester@example.com>")
+	_ = m.To("Ellenor Tester <ellinor@example.com>")
+	m.SetBodyString(TypeTextPlain, "This is a test")
+
+	curTmpDir := os.Getenv("TMPDIR")
+	defer func() {
+		if err := os.Setenv("TMPDIR", curTmpDir); err != nil {
+			t.Errorf("failed to set TMPDIR environment variable: %s", err)
+		}
+	}()
+
+	if err := os.Setenv("TMPDIR", "/invalid/directory/that/does/not/exist"); err != nil {
+		t.Errorf("failed to set TMPDIR environment variable: %s", err)
+	}
+	_, err := m.WriteToTempFile()
+	if err == nil {
+		t.Errorf("WriteToTempFile() did not fail as expected")
+	}
+}

msg_test.go

@@ -786,13 +786,11 @@ func TestMsg_SetMessageIDWithValue(t *testing.T) {
 // TestMsg_SetMessageIDRandomness tests the randomness of Msg.SetMessageID methods
 func TestMsg_SetMessageIDRandomness(t *testing.T) {
 	var mids []string
-	for i := 0; i < 100; i++ {
+	m := NewMsg()
-		m := NewMsg()
+	for i := 0; i < 50_000; i++ {
 		m.SetMessageID()
-		mid := m.GetGenHeader(HeaderMessageID)
+		mid := m.GetMessageID()
-		if len(mid) > 0 {
+		mids = append(mids, mid)
-			mids = append(mids, mid[0])
-		}
 	}
 	c := make(map[string]int)
 	for i := range mids {
@@ -805,6 +803,21 @@ func TestMsg_SetMessageIDRandomness(t *testing.T) {
 	}
 }
 
+func TestMsg_GetMessageID(t *testing.T) {
+	expected := "this.is.a.message.id"
+	msg := NewMsg()
+	msg.SetMessageIDWithValue(expected)
+	val := msg.GetMessageID()
+	if !strings.EqualFold(val, fmt.Sprintf("<%s>", expected)) {
+		t.Errorf("GetMessageID() failed. Expected: %s, got: %s", fmt.Sprintf("<%s>", expected), val)
+	}
+	msg.genHeader[HeaderMessageID] = nil
+	val = msg.GetMessageID()
+	if val != "" {
+		t.Errorf("GetMessageID() failed. Expected empty string, got: %s", val)
+	}
+}
+
 // TestMsg_FromFormat tests the FromFormat and EnvelopeFrom methods for the Msg object
 func TestMsg_FromFormat(t *testing.T) {
 	tests := []struct {
@@ -1911,6 +1924,22 @@ func TestMsg_hasAltWithSMime(t *testing.T) {
 	}
 }
 
+// TestMsg_hasSMime tests the hasSMime() method of the Msg
+func TestMsg_hasSMime(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	m := NewMsg()
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	m.SetBodyString(TypeTextPlain, "Plain")
+	if !m.hasSMime() {
+		t.Errorf("mail has smime configured but hasSMime() returned true")
+	}
+}
+
 // TestMsg_hasRelated tests the hasRelated() method of the Msg
 func TestMsg_hasRelated(t *testing.T) {
 	m := NewMsg()
@@ -1978,6 +2007,70 @@ func TestMsg_WriteToSkipMiddleware(t *testing.T) {
 	}
 }
 
+// TestMsg_WriteToWithSMIME tests the WriteTo() method of the Msg
+func TestMsg_WriteToWithSMIME(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+
+	m := NewMsg()
+	m.Subject("This is a test")
+	m.SetBodyString(TypeTextPlain, "Plain")
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+
+	wbuf := bytes.Buffer{}
+	if _, err = m.WriteTo(&wbuf); err != nil {
+		t.Errorf("WriteTo() failed: %s", err)
+	}
+
+	result := wbuf.String()
+	boundary := result[strings.LastIndex(result, "--")-60 : strings.LastIndex(result, "--")]
+	if strings.Count(result, boundary) != 4 {
+		t.Errorf("WriteTo() failed. False number of boundaries found")
+	}
+
+	parts := strings.Split(result, fmt.Sprintf("--%s", boundary))
+	if len(parts) != 4 {
+		t.Errorf("WriteTo() failed. False number of parts found")
+	}
+
+	preamble := parts[0]
+	if !strings.Contains(preamble, "MIME-Version: 1.0") {
+		t.Errorf("WriteTo() failed. Unable to find MIME-Version")
+	}
+	if !strings.Contains(preamble, "Subject: This is a test") {
+		t.Errorf("WriteTo() failed. Unable to find subject")
+	}
+	if !strings.Contains(preamble, fmt.Sprintf("Content-Type: multipart/signed; protocol=\"application/pkcs7-signature\"; micalg=sha-256;\r\n boundary=%s", boundary)) {
+		t.Errorf("WriteTo() failed. Unable to find Content-Type")
+	}
+
+	signedData := parts[1]
+	if !strings.Contains(signedData, "Content-Transfer-Encoding: quoted-printable") {
+		t.Errorf("WriteTo() failed. Unable to find Content-Transfer-Encoding")
+	}
+	if !strings.Contains(signedData, "Content-Type: text/plain; charset=UTF-8") {
+		t.Errorf("WriteTo() failed. Unable to find Content-Type")
+	}
+	if !strings.Contains(signedData, "Plain") {
+		t.Errorf("WriteTo() failed. Unable to find Content")
+	}
+
+	signature := parts[2]
+	if !strings.Contains(signature, "Content-Transfer-Encoding: base64") {
+		t.Errorf("WriteTo() failed. Unable to find Content-Transfer-Encoding")
+	}
+	if !strings.Contains(signature, `application/pkcs7-signature; name="smime.p7s"`) {
+		t.Errorf("WriteTo() failed. Unable to find Content-Type")
+	}
+	if strings.Contains(signature, "Plain") {
+		t.Errorf("WriteTo() failed. Unable to find signature")
+	}
+}
+
 // TestMsg_WriteTo_fails tests the WriteTo() method of the Msg but with a failing body writer function
 func TestMsg_WriteTo_fails(t *testing.T) {
 	m := NewMsg()
@@ -3266,9 +3359,6 @@ func TestSignWithSMime_ValidKeyPair(t *testing.T) {
 	if m.sMime.certificate == nil {
 		t.Errorf("WithSMimeSinging() - no certificate is given")
 	}
-	if len(m.sMime.parentCertificates) != len(keyPair.Certificate[:1]) {
-		t.Errorf("WithSMimeSinging() - no certificate is given")
-	}
 }
 
 // TestSignWithSMime_InvalidKeyPair tests WithSMimeSinging with given invalid key pair

msg_totmpfile.go

@@ -12,8 +12,14 @@ import (
 	"os"
 )
 
-// WriteToTempFile will create a temporary file and output the Msg to this file
+// WriteToTempFile creates a temporary file and writes the Msg content to this file.
-// The method will return the filename of the temporary file
+//
+// This method generates a temporary file with a ".eml" extension, writes the Msg to it, and returns the
+// filename of the created temporary file.
+//
+// Returns:
+//   - A string representing the filename of the temporary file.
+//   - An error if the file creation or writing process fails.
 func (m *Msg) WriteToTempFile() (string, error) {
 	f, err := os.CreateTemp("", "go-mail_*.eml")
 	if err != nil {

msg_totmpfile_116.go

@@ -12,8 +12,14 @@ import (
 	"io/ioutil"
 )
 
-// WriteToTempFile will create a temporary file and output the Msg to this file
+// WriteToTempFile creates a temporary file and writes the Msg content to this file.
-// The method will return the filename of the temporary file
+//
+// This method generates a temporary file with a ".eml" extension, writes the Msg to it, and returns the
+// filename of the created temporary file.
+//
+// Returns:
+//   - A string representing the filename of the temporary file.
+//   - An error if the file creation or writing process fails.
 func (m *Msg) WriteToTempFile() (string, error) {
 	f, err := ioutil.TempFile("", "go-mail_*.eml")
 	if err != nil {

msgwriter.go

@@ -18,22 +18,39 @@ import (
 	"strings"
 )
 
-// MaxHeaderLength defines the maximum line length for a mail header
+const (
-// RFC 2047 suggests 76 characters
+	// MaxHeaderLength defines the maximum line length for a mail header.
-const MaxHeaderLength = 76
+	//
+	// This constant follows the recommendation of RFC 2047, which suggests a maximum length of 76 characters.
+	//
+	// References:
+	//   - https://datatracker.ietf.org/doc/html/rfc2047
+	MaxHeaderLength = 76
 
-// MaxBodyLength defines the maximum line length for the mail body
+	// MaxBodyLength defines the maximum line length for the mail body.
-// RFC 2047 suggests 76 characters
+	//
-const MaxBodyLength = 76
+	// This constant follows the recommendation of RFC 2047, which suggests a maximum length of 76 characters.
+	//
+	// References:
+	//   - https://datatracker.ietf.org/doc/html/rfc2047
+	MaxBodyLength = 76
 
-// SingleNewLine represents a new line that can be used by the msgWriter to issue a carriage return
+	// SingleNewLine represents a single newline character sequence ("\r\n").
-const SingleNewLine = "\r\n"
+	//
+	// This constant can be used by the msgWriter to issue a carriage return when writing mail content.
+	SingleNewLine = "\r\n"
 
-// DoubleNewLine represents a double new line that can be used by the msgWriter to
+	// DoubleNewLine represents a double newline character sequence ("\r\n\r\n").
-// indicate a new segement of the mail
+	//
-const DoubleNewLine = "\r\n\r\n"
+	// This constant can be used by the msgWriter to indicate a new segment of the mail when writing mail content.
+	DoubleNewLine = "\r\n\r\n"
+)
 
-// msgWriter handles the I/O to the io.WriteCloser of the SMTP client
+// msgWriter handles the I/O operations for writing to the io.WriteCloser of the SMTP client.
+//
+// This struct keeps track of the number of bytes written, the character set used, and the depth of the
+// current multipart section. It also handles encoding, error tracking, and managing multipart and part
+// writers for constructing the email message body.
 type msgWriter struct {
 	bytesWritten    int64
 	charset         Charset
@@ -45,7 +62,18 @@ type msgWriter struct {
 	writer          io.Writer
 }
 
-// Write implements the io.Writer interface for msgWriter
+// Write implements the io.Writer interface for msgWriter.
+//
+// This method writes the provided payload to the underlying writer. It keeps track of the number of bytes
+// written and handles any errors encountered during the writing process. If a previous error exists, it
+// prevents further writing and returns the error.
+//
+// Parameters:
+//   - payload: A byte slice containing the data to be written.
+//
+// Returns:
+//   - The number of bytes successfully written.
+//   - An error if the writing process fails, or if a previous error was encountered.
 func (mw *msgWriter) Write(payload []byte) (int, error) {
 	if mw.err != nil {
 		return 0, fmt.Errorf("failed to write due to previous error: %w", mw.err)
@@ -57,7 +85,19 @@ func (mw *msgWriter) Write(payload []byte) (int, error) {
 	return n, mw.err
 }
 
-// writeMsg formats the message and sends it to its io.Writer
+// writeMsg formats the message and writes it to the msgWriter's io.Writer.
+//
+// This method handles the process of writing the message headers and body content, including handling
+// multipart structures (e.g., mixed, related, alternative), PGP types, and attachments/embeds. It sets the
+// required headers (e.g., "From", "To", "Cc") and iterates over the message parts, writing them to the
+// output writer.
+//
+// Parameters:
+//   - msg: A pointer to the Msg struct containing the message data and headers to be written.
+//
+// References:
+//   - https://datatracker.ietf.org/doc/html/rfc2045 (Multipurpose Internet Mail Extensions - MIME)
+//   - https://datatracker.ietf.org/doc/html/rfc5322 (Internet Message Format)
 func (mw *msgWriter) writeMsg(msg *Msg) {
 	msg.addDefaultHeader()
 	msg.checkUserAgent()
@@ -144,7 +184,13 @@ func (mw *msgWriter) writeMsg(msg *Msg) {
 	}
 }
 
-// writeGenHeader writes out all generic headers to the msgWriter
+// writeGenHeader writes out all generic headers to the msgWriter.
+//
+// This function extracts all generic headers from the provided Msg object, sorts them, and writes them
+// to the msgWriter in alphabetical order.
+//
+// Parameters:
+//   - msg: The Msg object containing the headers to be written.
 func (mw *msgWriter) writeGenHeader(msg *Msg) {
 	keys := make([]string, 0, len(msg.genHeader))
 	for key := range msg.genHeader {
@@ -156,14 +202,32 @@ func (mw *msgWriter) writeGenHeader(msg *Msg) {
 	}
 }
 
-// writePreformatedHeader writes out all preformated generic headers to the msgWriter
+// writePreformattedGenHeader writes out all preformatted generic headers to the msgWriter.
+//
+// This function iterates over all preformatted generic headers from the provided Msg object and writes
+// them to the msgWriter in the format "key: value" followed by a newline.
+//
+// Parameters:
+//   - msg: The Msg object containing the preformatted headers to be written.
 func (mw *msgWriter) writePreformattedGenHeader(msg *Msg) {
 	for key, val := range msg.preformHeader {
 		mw.writeString(fmt.Sprintf("%s: %s%s", key, val, SingleNewLine))
 	}
 }
 
-// startMP writes a multipart beginning
+// startMP writes a multipart beginning.
+//
+// This function initializes a multipart writer for the msgWriter using the specified MIME type and
+// boundary. It sets the Content-Type header to indicate the multipart type and writes the boundary
+// information. If a boundary is provided, it is set explicitly; otherwise, a default boundary is
+// generated. It also handles writing a new part when nested multipart structures are used.
+//
+// Parameters:
+//   - mimeType: The MIME type of the multipart content (e.g., "mixed", "alternative").
+//   - boundary: The boundary string separating different parts of the multipart message.
+//
+// References:
+//   - https://datatracker.ietf.org/doc/html/rfc2046
 func (mw *msgWriter) startMP(mimeType MIMEType, boundary string) {
 	multiPartWriter := multipart.NewWriter(mw)
 	if boundary != "" {
@@ -183,7 +247,10 @@ func (mw *msgWriter) startMP(mimeType MIMEType, boundary string) {
 	mw.depth++
 }
 
-// stopMP closes the multipart
+// stopMP closes the multipart.
+//
+// This function closes the current multipart writer if there is an active multipart structure.
+// It decreases the depth level of multipart nesting.
 func (mw *msgWriter) stopMP() {
 	if mw.depth > 0 {
 		mw.err = mw.multiPartWriter[mw.depth-1].Close()
@@ -191,7 +258,17 @@ func (mw *msgWriter) stopMP() {
 	}
 }
 
-// addFiles adds the attachments/embeds file content to the mail body
+// addFiles adds the attachments/embeds file content to the mail body.
+//
+// This function iterates through the list of files, setting necessary headers for each file,
+// including Content-Type, Content-Transfer-Encoding, Content-Disposition, and Content-ID
+// (if the file is an embed). It determines the appropriate MIME type for each file based on
+// its extension or the provided ContentType. It writes file headers and file content
+// to the mail body using the appropriate encoding.
+//
+// Parameters:
+//   - files: A slice of File objects to be added to the mail body.
+//   - isAttachment: A boolean indicating whether the files are attachments (true) or embeds (false).
 func (mw *msgWriter) addFiles(files []*File, isAttachment bool) {
 	for _, file := range files {
 		encoding := EncodingB64
@@ -250,18 +327,40 @@ func (mw *msgWriter) addFiles(files []*File, isAttachment bool) {
 	}
 }
 
-// newPart creates a new MIME multipart io.Writer and sets the partwriter to it
+// newPart creates a new MIME multipart io.Writer and sets the partWriter to it.
+//
+// This function creates a new MIME part using the provided header information and assigns it
+// to the partWriter. It interacts with the current multipart writer at the specified depth
+// to create the part.
+//
+// Parameters:
+//   - header: A map containing the header fields and their corresponding values for the new part.
 func (mw *msgWriter) newPart(header map[string][]string) {
 	mw.partWriter, mw.err = mw.multiPartWriter[mw.depth-1].CreatePart(header)
 }
 
-// writePart writes the corresponding part to the Msg body
+// writePart writes the corresponding part to the Msg body.
+//
+// This function writes a MIME part to the message body, setting the appropriate headers such
+// as Content-Type and Content-Transfer-Encoding. It determines the charset for the part,
+// either using the part's own charset or a fallback charset if none is specified. If the part
+// is at the top level (depth 0), headers are written directly. For nested parts, it creates
+// a new MIME part with the provided headers.
+//
+// Parameters:
+//   - part: The Part object containing the data to be written.
+//   - charset: The Charset used as a fallback if the part does not specify one.
 func (mw *msgWriter) writePart(part *Part, charset Charset) {
 	partCharset := part.charset
 	if partCharset.String() == "" {
 		partCharset = charset
 	}
-	contentType := fmt.Sprintf("%s; charset=%s", part.contentType, partCharset)
+
+	contentType := part.contentType.String()
+	if !part.IsSMimeSigned() {
+		contentType = strings.Join([]string{contentType, "; charset=", partCharset.String()}, "")
+	}
+
 	contentTransferEnc := part.encoding.String()
 	if mw.depth == 0 {
 		mw.writeHeader(HeaderContentType, contentType)
@@ -280,7 +379,14 @@ func (mw *msgWriter) writePart(part *Part, charset Charset) {
 	mw.writeBody(part.writeFunc, part.encoding, part.smime)
 }
 
-// writeString writes a string into the msgWriter's io.Writer interface
+// writeString writes a string into the msgWriter's io.Writer interface.
+//
+// This function writes the given string to the msgWriter's underlying writer. It checks for
+// existing errors before performing the write operation. It also tracks the number of bytes
+// written and updates the bytesWritten field accordingly.
+//
+// Parameters:
+//   - s: The string to be written.
 func (mw *msgWriter) writeString(s string) {
 	if mw.err != nil {
 		return
@@ -290,7 +396,16 @@ func (mw *msgWriter) writeString(s string) {
 	mw.bytesWritten += int64(n)
 }
 
-// writeHeader writes a header into the msgWriter's io.Writer
+// writeHeader writes a header into the msgWriter's io.Writer.
+//
+// This function writes a header key and its associated values to the msgWriter. It ensures
+// proper formatting of long headers by inserting line breaks as needed. The header values
+// are joined and split into words to ensure compliance with the maximum header length
+// (MaxHeaderLength). After processing the header, it is written to the underlying writer.
+//
+// Parameters:
+//   - key: The Header key to be written.
+//   - values: A variadic parameter representing the values associated with the header.
 func (mw *msgWriter) writeHeader(key Header, values ...string) {
 	buffer := strings.Builder{}
 	charLength := MaxHeaderLength - 2
@@ -325,7 +440,18 @@ func (mw *msgWriter) writeHeader(key Header, values ...string) {
 	mw.writeString("\r\n")
 }
 
-// writeBody writes an io.Reader into an io.Writer using provided Encoding
+// writeBody writes an io.Reader into an io.Writer using the provided Encoding.
+//
+// This function writes data from an io.Reader to the underlying writer using a specified
+// encoding (quoted-printable, base64, or no encoding). It handles encoding of the content
+// and manages writing the encoded data to the appropriate writer, depending on the depth
+// (whether the data is part of a multipart structure or not). It also tracks the number
+// of bytes written and manages any errors encountered during the process.
+//
+// Parameters:
+//   - writeFunc: A function that writes the body content to the given io.Writer.
+//   - encoding: The encoding type to use when writing the content (e.g., base64, quoted-printable).
+//   - singingWithSMime: Whether the msg should be signed with S/MIME or not.
 func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encoding Encoding, singingWithSMime bool) {
 	var writer io.Writer
 	var encodedWriter io.WriteCloser

msgwriter_test.go

@@ -161,6 +161,7 @@ func TestMsgWriter_writeMsg_SMime(t *testing.T) {
 	if err != nil {
 		t.Errorf("failed to load dummy certificate. Cause: %v", err)
 	}
+
 	m := NewMsg()
 	if err := m.SignWithSMime(keyPair); err != nil {
 		t.Errorf("set of certificate was not successful")
@@ -173,7 +174,22 @@ func TestMsgWriter_writeMsg_SMime(t *testing.T) {
 	mw := &msgWriter{writer: &buf, charset: CharsetUTF8, encoder: mime.QEncoding}
 	mw.writeMsg(m)
 	ms := buf.String()
-	if !strings.Contains(ms, `multipart/signed; protocol="application/pkcs7-signature"; micalg=sha256;`) {
+
-		t.Errorf("writeMsg failed. Expected PGP encoding header but didn't find it in message output")
+	if !strings.Contains(ms, "MIME-Version: 1.0") {
+		t.Errorf("writeMsg failed. Unable to find MIME-Version")
+	}
+	if !strings.Contains(ms, "Subject: This is a subject") {
+		t.Errorf("writeMsg failed. Unable to find subject")
+	}
+	if !strings.Contains(ms, "From: \"Toni Tester\" <test@example.com>") {
+		t.Errorf("writeMsg failed. Unable to find transmitter")
+	}
+	if !strings.Contains(ms, "To: \"Toni Receiver\" <receiver@example.com>") {
+		t.Errorf("writeMsg failed. Unable to find receiver")
+	}
+
+	boundary := ms[strings.LastIndex(ms, "--")-60 : strings.LastIndex(ms, "--")]
+	if !strings.Contains(ms, fmt.Sprintf("Content-Type: multipart/signed; protocol=\"application/pkcs7-signature\"; micalg=sha-256;\r\n boundary=%s", boundary)) {
+		t.Errorf("writeMsg failed. Unable to find Content-Type")
 	}
 }

part.go

@@ -12,7 +12,11 @@ import (
 // PartOption returns a function that can be used for grouping Part options
 type PartOption func(*Part)
 
-// Part is a part of the Msg
+// Part is a part of the Msg.
+//
+// This struct represents a single part of a multipart message. Each part has a content type,
+// charset, optional description, encoding, and a function to write its content to an io.Writer.
+// It also includes a flag to mark the part as deleted.
 type Part struct {
 	contentType ContentType
 	charset     Charset
@@ -23,7 +27,14 @@ type Part struct {
 	smime       bool
 }
 
-// GetContent executes the WriteFunc of the Part and returns the content as byte slice
+// GetContent executes the WriteFunc of the Part and returns the content as a byte slice.
+//
+// This function runs the part's writeFunc to write its content into a buffer and then returns
+// the content as a byte slice. If an error occurs during the writing process, it is returned.
+//
+// Returns:
+//   - A byte slice containing the part's content.
+//   - An error if the writeFunc encounters an issue.
 func (p *Part) GetContent() ([]byte, error) {
 	var b bytes.Buffer
 	if _, err := p.writeFunc(&b); err != nil {
@@ -32,27 +43,54 @@ func (p *Part) GetContent() ([]byte, error) {
 	return b.Bytes(), nil
 }
 
-// GetCharset returns the currently set Charset of the Part
+// GetCharset returns the currently set Charset of the Part.
+//
+// This function returns the Charset that is currently set for the Part.
+//
+// Returns:
+//   - The Charset of the Part.
 func (p *Part) GetCharset() Charset {
 	return p.charset
 }
 
-// GetContentType returns the currently set ContentType of the Part
+// GetContentType returns the currently set ContentType of the Part.
+//
+// This function returns the ContentType that is currently set for the Part.
+//
+// Returns:
+//   - The ContentType of the Part.
 func (p *Part) GetContentType() ContentType {
 	return p.contentType
 }
 
-// GetEncoding returns the currently set Encoding of the Part
+// GetEncoding returns the currently set Encoding of the Part.
+//
+// This function returns the Encoding that is currently set for the Part.
+//
+// Returns:
+//   - The Encoding of the Part.
 func (p *Part) GetEncoding() Encoding {
 	return p.encoding
 }
 
-// GetWriteFunc returns the currently set WriterFunc of the Part
+// GetWriteFunc returns the currently set WriteFunc of the Part.
+//
+// This function returns the WriteFunc that is currently set for the Part, which writes
+// the part's content to an io.Writer.
+//
+// Returns:
+//   - The WriteFunc of the Part, which is a function that takes an io.Writer and returns
+//     the number of bytes written and an error (if any).
 func (p *Part) GetWriteFunc() func(io.Writer) (int64, error) {
 	return p.writeFunc
 }
 
-// GetDescription returns the currently set Content-Description of the Part
+// GetDescription returns the currently set Content-Description of the Part.
+//
+// This function returns the Content-Description that is currently set for the Part.
+//
+// Returns:
+//   - The Content-Description of the Part as a string.
 func (p *Part) GetDescription() string {
 	return p.description
 }
@@ -62,63 +100,126 @@ func (p *Part) IsSMimeSigned() bool {
 	return p.smime
 }
 
-// SetContent overrides the content of the Part with the given string
+// SetContent overrides the content of the Part with the given string.
+//
+// This function sets the content of the Part by creating a new writeFunc that writes the
+// provided string content to an io.Writer.
+//
+// Parameters:
+//   - content: The string that will replace the current content of the Part.
 func (p *Part) SetContent(content string) {
 	buffer := bytes.NewBufferString(content)
 	p.writeFunc = writeFuncFromBuffer(buffer)
 }
 
-// SetContentType overrides the ContentType of the Part
+// SetContentType overrides the ContentType of the Part.
+//
+// This function sets a new ContentType for the Part, replacing the existing one.
+//
+// Parameters:
+//   - contentType: The new ContentType to be set for the Part.
 func (p *Part) SetContentType(contentType ContentType) {
 	p.contentType = contentType
 }
 
-// SetCharset overrides the Charset of the Part
+// SetCharset overrides the Charset of the Part.
+//
+// This function sets a new Charset for the Part, replacing the existing one.
+//
+// Parameters:
+//   - charset: The new Charset to be set for the Part.
 func (p *Part) SetCharset(charset Charset) {
 	p.charset = charset
 }
 
-// SetEncoding creates a new mime.WordEncoder based on the encoding setting of the message
+// SetEncoding creates a new mime.WordEncoder based on the encoding setting of the message.
+//
+// This function sets a new Encoding for the Part, replacing the existing one.
+//
+// Parameters:
+//   - encoding: The new Encoding to be set for the Part.
 func (p *Part) SetEncoding(encoding Encoding) {
 	p.encoding = encoding
 }
 
-// SetDescription overrides the Content-Description of the Part
+// SetDescription overrides the Content-Description of the Part.
+//
+// This function sets a new Content-Description for the Part, replacing the existing one.
+//
+// Parameters:
+//   - description: The new Content-Description to be set for the Part.
 func (p *Part) SetDescription(description string) {
 	p.description = description
 }
 
+
 // SetIsSMimeSigned sets the flag for signing the Part with S/MIME
 func (p *Part) SetIsSMimeSigned(smime bool) {
 	p.smime = smime
 }
 
-// SetWriteFunc overrides the WriteFunc of the Part
+// SetWriteFunc overrides the WriteFunc of the Part.
+//
+// This function sets a new WriteFunc for the Part, replacing the existing one. The WriteFunc
+// is responsible for writing the Part's content to an io.Writer.
+//
+// Parameters:
+//   - writeFunc: A function that writes the Part's content to an io.Writer and returns
+//     the number of bytes written and an error (if any).
 func (p *Part) SetWriteFunc(writeFunc func(io.Writer) (int64, error)) {
 	p.writeFunc = writeFunc
 }
 
-// Delete removes the current part from the parts list of the Msg by setting the
+// Delete removes the current part from the parts list of the Msg by setting the isDeleted flag to true.
-// isDeleted flag to true. The msgWriter will skip it then
+//
+// This function marks the Part as deleted by setting the isDeleted flag to true. The msgWriter
+// will skip over this Part during processing.
 func (p *Part) Delete() {
 	p.isDeleted = true
 }
 
-// WithPartCharset overrides the default Part charset
+// WithPartCharset overrides the default Part charset.
+//
+// This function returns a PartOption that allows the charset of a Part to be overridden
+// with the specified Charset.
+//
+// Parameters:
+//   - charset: The Charset to be set for the Part.
+//
+// Returns:
+//   - A PartOption function that sets the Part's charset.
 func WithPartCharset(charset Charset) PartOption {
 	return func(p *Part) {
 		p.charset = charset
 	}
 }
 
-// WithPartEncoding overrides the default Part encoding
+// WithPartEncoding overrides the default Part encoding.
+//
+// This function returns a PartOption that allows the encoding of a Part to be overridden
+// with the specified Encoding.
+//
+// Parameters:
+//   - encoding: The Encoding to be set for the Part.
+//
+// Returns:
+//   - A PartOption function that sets the Part's encoding.
 func WithPartEncoding(encoding Encoding) PartOption {
 	return func(p *Part) {
 		p.encoding = encoding
 	}
 }
 
-// WithPartContentDescription overrides the default Part Content-Description
+// WithPartContentDescription overrides the default Part Content-Description.
+//
+// This function returns a PartOption that allows the Content-Description of a Part
+// to be overridden with the specified description.
+//
+// Parameters:
+//   - description: The Content-Description to be set for the Part.
+//
+// Returns:
+//   - A PartOption function that sets the Part's Content-Description.
 func WithPartContentDescription(description string) PartOption {
 	return func(p *Part) {
 		p.description = description

random.go

@@ -7,23 +7,41 @@ package mail
 import (
 	"crypto/rand"
 	"encoding/binary"
-	"fmt"
-	"math/big"
 	"strings"
 )
 
 // Range of characters for the secure string generation
-const cr = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
+const cr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789._-"
 
 // Bitmask sizes for the string generators (based on 93 chars total)
+//
+// These constants define bitmask-related values used for efficient random string generation.
+// The bitmask operates over 66 possible characters, and the constants help determine the
+// number of bits and indices used in the process.
 const (
-	letterIdxBits = 7                    // 7 bits to represent a letter index
+	// letterIdxBits: Number of bits needed to represent a letter index. We have 64 possible characters
-	letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
+	// which fit into 6 bits.
-	letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
+	letterIdxBits = 6
+	// letterIdxMask: Bitmask to extract letter indices (all 1-bits for letterIdxBits).
+	letterIdxMask = 1<<letterIdxBits - 1
+	// letterIdxMax: The maximum number of letter indices that fit in 63 bits.
+	letterIdxMax = 63 / letterIdxBits
 )
 
-// randomStringSecure returns a random, string of length characters. This method uses the
+// randomStringSecure returns a random string of the specified length.
-// crypto/random package and therfore is cryptographically secure
+//
+// This function generates a cryptographically secure random string of the given length using
+// the crypto/rand package. It ensures that the randomness is secure and suitable for
+// cryptographic purposes. The function reads random bytes, converts them to indices within
+// a character range, and builds the string. If an error occurs while reading from the random
+// pool, it returns the error.
+//
+// Parameters:
+//   - length: The length of the random string to be generated.
+//
+// Returns:
+//   - A randomly generated string.
+//   - An error if the random generation fails.
 func randomStringSecure(length int) (string, error) {
 	randString := strings.Builder{}
 	randString.Grow(length)
@@ -52,23 +70,3 @@ func randomStringSecure(length int) (string, error) {
 
 	return randString.String(), nil
 }
-
-// randNum returns a random number with a maximum value of length
-func randNum(length int) (int, error) {
-	if length <= 0 {
-		return 0, fmt.Errorf("provided number is <= 0: %d", length)
-	}
-	length64 := big.NewInt(int64(length))
-	if !length64.IsUint64() {
-		return 0, fmt.Errorf("big.NewInt() generation returned negative value: %d", length64)
-	}
-	randNum64, err := rand.Int(rand.Reader, length64)
-	if err != nil {
-		return 0, err
-	}
-	randomNum := int(randNum64.Int64())
-	if randomNum < 0 {
-		return 0, fmt.Errorf("generated random number does not fit as int64: %d", randNum64)
-	}
-	return randomNum, nil
-}

random_test.go

@@ -38,33 +38,12 @@ func TestRandomStringSecure(t *testing.T) {
 	}
 }
 
-// TestRandomNum tests the randomNum method
+func BenchmarkGenerator_RandomStringSecure(b *testing.B) {
-func TestRandomNum(t *testing.T) {
+	b.ReportAllocs()
-	tt := []struct {
+	for i := 0; i < b.N; i++ {
-		testName string
+		_, err := randomStringSecure(22)
-		max      int
+		if err != nil {
-	}{
+			b.Errorf("RandomStringFromCharRange() failed: %s", err)
-		{"Max: 1", 1},
+		}
-		{"Max: 20", 20},
-		{"Max: 50", 50},
-		{"Max: 100", 100},
-		{"Max: 1000", 1000},
-		{"Max: 10000", 10000},
-		{"Max: 100000000", 100000000},
-	}
-
-	for _, tc := range tt {
-		t.Run(tc.testName, func(t *testing.T) {
-			rn, err := randNum(tc.max)
-			if err != nil {
-				t.Errorf("random number generation failed: %s", err)
-			}
-			if rn < 0 {
-				t.Errorf("random number generation failed: %d is smaller than zero", rn)
-			}
-			if rn > tc.max {
-				t.Errorf("random number generation failed: %d is bigger than given value %d", rn, tc.max)
-			}
-		})
 	}
 }

reader.go

@@ -8,19 +8,41 @@ import (
 	"io"
 )
 
-// Reader is a type that implements the io.Reader interface for a Msg
+// Reader is a type that implements the io.Reader interface for a Msg.
+//
+// This struct represents a reader that reads from a byte slice buffer. It keeps track of the
+// current read position (offset) and any initialization error. The buffer holds the data to be
+// read from the message.
 type Reader struct {
 	buffer []byte // contents are the bytes buffer[offset : len(buffer)]
 	offset int    // read at &buffer[offset], write at &buffer[len(buffer)]
 	err    error  // initialization error
 }
 
-// Error returns an error if the Reader err field is not nil
+// Error returns an error if the Reader err field is not nil.
+//
+// This function checks the Reader's err field and returns it if it is not nil. If no error
+// occurred during initialization, it returns nil.
+//
+// Returns:
+//   - The error stored in the err field, or nil if no error is present.
 func (r *Reader) Error() error {
 	return r.err
 }
 
-// Read reads the length of p of the Msg buffer to satisfy the io.Reader interface
+// Read reads the content of the Msg buffer into the provided payload to satisfy the io.Reader interface.
+//
+// This function reads data from the Reader's buffer into the provided byte slice (payload).
+// It checks for errors or an empty buffer and resets the Reader if necessary. If no data is available,
+// it returns io.EOF. Otherwise, it copies the content from the buffer into the payload and updates
+// the read offset.
+//
+// Parameters:
+//   - payload: A byte slice where the data will be copied.
+//
+// Returns:
+//   - n: The number of bytes copied into the payload.
+//   - err: An error if any issues occurred during the read operation or io.EOF if the buffer is empty.
 func (r *Reader) Read(payload []byte) (n int, err error) {
 	if r.err != nil {
 		return 0, r.err
@@ -37,12 +59,20 @@ func (r *Reader) Read(payload []byte) (n int, err error) {
 	return n, err
 }
 
-// Reset resets the Reader buffer to be empty, but it retains the underlying storage
+// Reset resets the Reader buffer to be empty, but it retains the underlying storage for future use.
-// for use by future writes.
+//
+// This function clears the Reader's buffer by setting its length to 0 and resets the read offset
+// to the beginning. The underlying storage is retained, allowing future writes to reuse the buffer.
 func (r *Reader) Reset() {
 	r.buffer = r.buffer[:0]
 	r.offset = 0
 }
 
 // empty reports whether the unread portion of the Reader buffer is empty.
+//
+// This function checks if the unread portion of the Reader's buffer is empty by comparing
+// the length of the buffer to the current read offset.
+//
+// Returns:
+//   - true if the unread portion is empty, false otherwise.
 func (r *Reader) empty() bool { return len(r.buffer) <= r.offset }

senderror.go

@@ -54,18 +54,32 @@ const (
 	ErrAmbiguous
 )
 
-// SendError is an error wrapper for delivery errors of the Msg
+// SendError is an error wrapper for delivery errors of the Msg.
+//
+// This struct represents an error that occurs during the delivery of a message. It holds
+// details about the affected message, a list of errors, the recipient list, and whether
+// the error is temporary or permanent. It also includes a reason code for the error.
 type SendError struct {
-	Reason  SendErrReason
+	affectedMsg *Msg
-	isTemp  bool
+	errlist     []error
-	errlist []error
+	isTemp      bool
-	rcpt    []string
+	rcpt        []string
+	Reason      SendErrReason
 }
 
 // SendErrReason represents a comparable reason on why the delivery failed
 type SendErrReason int
 
-// Error implements the error interface for the SendError type
+// Error implements the error interface for the SendError type.
+//
+// This function returns a detailed error message string for the SendError, including the
+// reason for failure, list of errors, affected recipients, and the message ID of the
+// affected message (if available). If the reason is unknown (greater than 10), it returns
+// "unknown reason". The error message is built dynamically based on the content of the
+// error list, recipient list, and message ID.
+//
+// Returns:
+//   - A string representing the error message.
 func (e *SendError) Error() string {
 	if e.Reason > 10 {
 		return "unknown reason"
@@ -92,10 +106,25 @@ func (e *SendError) Error() string {
 			}
 		}
 	}
+	if e.affectedMsg != nil && e.affectedMsg.GetMessageID() != "" {
+		errMessage.WriteString(", affected message ID: ")
+		errMessage.WriteString(e.affectedMsg.GetMessageID())
+	}
+
 	return errMessage.String()
 }
 
-// Is implements the errors.Is functionality and compares the SendErrReason
+// Is implements the errors.Is functionality and compares the SendErrReason.
+//
+// This function allows for comparison between two errors by checking if the provided
+// error matches the SendError type and, if so, compares the SendErrReason and the
+// temporary status (isTemp) of both errors.
+//
+// Parameters:
+//   - errType: The error to compare against the current SendError.
+//
+// Returns:
+//   - true if the errors have the same reason and temporary status, false otherwise.
 func (e *SendError) Is(errType error) bool {
 	var t *SendError
 	if errors.As(errType, &t) && t != nil {
@@ -104,7 +133,13 @@ func (e *SendError) Is(errType error) bool {
 	return false
 }
 
-// IsTemp returns true if the delivery error is of temporary nature and can be retried
+// IsTemp returns true if the delivery error is of a temporary nature and can be retried.
+//
+// This function checks whether the SendError indicates a temporary error, which suggests
+// that the delivery can be retried. If the SendError is nil, it returns false.
+//
+// Returns:
+//   - true if the error is temporary, false otherwise.
 func (e *SendError) IsTemp() bool {
 	if e == nil {
 		return false
@@ -112,7 +147,42 @@ func (e *SendError) IsTemp() bool {
 	return e.isTemp
 }
 
-// String implements the Stringer interface for the SendErrReason
+// MessageID returns the message ID of the affected Msg that caused the error.
+//
+// This function retrieves the message ID of the Msg associated with the SendError.
+// If no message ID was set or if the SendError or Msg is nil, it returns an empty string.
+//
+// Returns:
+//   - The message ID as a string, or an empty string if no ID is available.
+func (e *SendError) MessageID() string {
+	if e == nil || e.affectedMsg == nil {
+		return ""
+	}
+	return e.affectedMsg.GetMessageID()
+}
+
+// Msg returns the pointer to the affected message that caused the error.
+//
+// This function retrieves the Msg associated with the SendError. If the SendError or
+// the affectedMsg is nil, it returns nil.
+//
+// Returns:
+//   - A pointer to the Msg that caused the error, or nil if not available.
+func (e *SendError) Msg() *Msg {
+	if e == nil || e.affectedMsg == nil {
+		return nil
+	}
+	return e.affectedMsg
+}
+
+// String satisfies the fmt.Stringer interface for the SendErrReason type.
+//
+// This function converts the SendErrReason into a human-readable string representation based
+// on the error type. If the error reason does not match any predefined case, it returns
+// "unknown reason".
+//
+// Returns:
+//   - A string representation of the SendErrReason.
 func (r SendErrReason) String() string {
 	switch r {
 	case ErrGetSender:
@@ -141,8 +211,16 @@ func (r SendErrReason) String() string {
 	return "unknown reason"
 }
 
-// isTempError checks the given SMTP error and returns true if the given error is of temporary nature
+// isTempError checks if the given SMTP error is of a temporary nature and should be retried.
-// and should be retried
+//
+// This function inspects the error message and returns true if the first character of the
+// error message is '4', indicating a temporary SMTP error that can be retried.
+//
+// Parameters:
+//   - err: The error to check.
+//
+// Returns:
+//   - true if the error is temporary, false otherwise.
 func isTempError(err error) bool {
 	return err.Error()[0] == '4'
 }

senderror_test.go

@@ -83,7 +83,96 @@ func TestSendError_IsTemp(t *testing.T) {
 	}
 }
 
+func TestSendError_IsTempNil(t *testing.T) {
+	var se *SendError
+	if se.IsTemp() {
+		t.Error("expected false on nil-senderror")
+	}
+}
+
+func TestSendError_MessageID(t *testing.T) {
+	var se *SendError
+	err := returnSendError(ErrAmbiguous, false)
+	if !errors.As(err, &se) {
+		t.Errorf("error mismatch, expected error to be of type *SendError")
+		return
+	}
+	if errors.As(err, &se) {
+		if se.MessageID() == "" {
+			t.Errorf("sendError expected message-id, but got empty string")
+		}
+		if !strings.EqualFold(se.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("sendError message-id expected: %s, but got: %s", "<this.is.a.message.id>",
+				se.MessageID())
+		}
+	}
+}
+
+func TestSendError_MessageIDNil(t *testing.T) {
+	var se *SendError
+	if se.MessageID() != "" {
+		t.Error("expected empty string on nil-senderror")
+	}
+}
+
+func TestSendError_Msg(t *testing.T) {
+	var se *SendError
+	err := returnSendError(ErrAmbiguous, false)
+	if !errors.As(err, &se) {
+		t.Errorf("error mismatch, expected error to be of type *SendError")
+		return
+	}
+	if errors.As(err, &se) {
+		if se.Msg() == nil {
+			t.Errorf("sendError expected msg pointer, but got nil")
+		}
+		from := se.Msg().GetFromString()
+		if len(from) == 0 {
+			t.Errorf("sendError expected msg from, but got empty string")
+			return
+		}
+		if !strings.EqualFold(from[0], "<toni.tester@domain.tld>") {
+			t.Errorf("sendError message from expected: %s, but got: %s", "<toni.tester@domain.tld>",
+				from[0])
+		}
+	}
+}
+
+func TestSendError_MsgNil(t *testing.T) {
+	var se *SendError
+	if se.Msg() != nil {
+		t.Error("expected nil on nil-senderror")
+	}
+}
+
+func TestSendError_IsFail(t *testing.T) {
+	err1 := returnSendError(ErrAmbiguous, false)
+	err2 := returnSendError(ErrSMTPMailFrom, false)
+	if errors.Is(err1, err2) {
+		t.Errorf("error mismatch, ErrAmbiguous should not be equal to ErrSMTPMailFrom")
+	}
+}
+
+func TestSendError_ErrorMulti(t *testing.T) {
+	expected := `ambiguous reason, check Msg.SendError for message specific reasons, ` +
+		`affected recipient(s): <email1@domain.tld>, <email2@domain.tld>`
+	err := &SendError{
+		Reason: ErrAmbiguous, isTemp: false, affectedMsg: nil,
+		rcpt: []string{"<email1@domain.tld>", "<email2@domain.tld>"},
+	}
+	if err.Error() != expected {
+		t.Errorf("error mismatch, expected: %s, got: %s", expected, err.Error())
+	}
+}
+
 // returnSendError is a helper method to retunr a SendError with a specific reason
 func returnSendError(r SendErrReason, t bool) error {
-	return &SendError{Reason: r, isTemp: t}
+	message := NewMsg()
+	_ = message.From("toni.tester@domain.tld")
+	_ = message.To("tina.tester@domain.tld")
+	message.Subject("This is the subject")
+	message.SetBodyString(TypeTextPlain, "This is the message body")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+
+	return &SendError{Reason: r, isTemp: t, affectedMsg: message}
 }

smime.go

@@ -16,9 +16,6 @@ var (
 	// ErrInvalidKeyPair should be used if key pair is invalid
 	ErrInvalidKeyPair = errors.New("invalid key pair")
 
-	// ErrInvalidCertificate should be used if a certificate is invalid
-	ErrInvalidCertificate = errors.New("invalid certificate")
-
 	// ErrCouldNotInitialize should be used if the signed data could not initialize
 	ErrCouldNotInitialize = errors.New("could not initialize signed data")
 
@@ -34,9 +31,8 @@ var (
 
 // SMime is used to sign messages with S/MIME
 type SMime struct {
-	privateKey         *rsa.PrivateKey
+	privateKey  *rsa.PrivateKey
-	certificate        *x509.Certificate
+	certificate *x509.Certificate
-	parentCertificates []*x509.Certificate
 }
 
 // NewSMime construct a new instance of SMime with a provided *tls.Certificate
@@ -45,53 +41,44 @@ func newSMime(keyPair *tls.Certificate) (*SMime, error) {
 		return nil, ErrInvalidKeyPair
 	}
 
-	parentCertificates := make([]*x509.Certificate, 0)
-	for _, cert := range keyPair.Certificate[1:] {
-		c, err := x509.ParseCertificate(cert)
-		if err != nil {
-			return nil, ErrInvalidCertificate
-		}
-		parentCertificates = append(parentCertificates, c)
-	}
-
 	return &SMime{
-		privateKey:         keyPair.PrivateKey.(*rsa.PrivateKey),
+		privateKey:  keyPair.PrivateKey.(*rsa.PrivateKey),
-		certificate:        keyPair.Leaf,
+		certificate: keyPair.Leaf,
-		parentCertificates: parentCertificates,
 	}, nil
 }
 
-// sign with the S/MIME method the message of the actual *Part
+// signMessage signs the message with S/MIME
-func (sm *SMime) sign(signaturePart *Part, message string) error {
+func (sm *SMime) signMessage(message string) (*string, error) {
 	lines := parseLines([]byte(message))
 	toBeSigned := lines.bytesFromLines([]byte("\r\n"))
 
-	tmp, err := pkcs7.NewSignedData(toBeSigned)
+	signedData, err := pkcs7.NewSignedData(toBeSigned)
-	tmp.SetDigestAlgorithm(pkcs7.OIDDigestAlgorithmSHA256)
+	signedData.SetDigestAlgorithm(pkcs7.OIDDigestAlgorithmSHA256)
 	if err != nil {
-		return ErrCouldNotInitialize
+		return nil, ErrCouldNotInitialize
 	}
 
-	if err = tmp.AddSignerChain(sm.certificate, sm.privateKey, sm.parentCertificates, pkcs7.SignerInfoConfig{}); err != nil {
+	if err = signedData.AddSigner(sm.certificate, sm.privateKey, pkcs7.SignerInfoConfig{}); err != nil {
-		return ErrCouldNotAddSigner
+		return nil, ErrCouldNotAddSigner
 	}
 
-	signatureDER, err := tmp.Finish()
+	signedData.Detach()
+
+	signatureDER, err := signedData.Finish()
 	if err != nil {
-		return ErrCouldNotFinishSigning
+		return nil, ErrCouldNotFinishSigning
 	}
 
 	pemMsg, err := encodeToPEM(signatureDER)
 	if err != nil {
-		return ErrCouldNoEncodeToPEM
+		return nil, ErrCouldNoEncodeToPEM
 	}
-	signaturePart.SetContent(*pemMsg)
 
-	return nil
+	return pemMsg, nil
 }
 
 // createMessage prepares the message that will be used for the sign method later
-func (sm *SMime) createMessage(encoding Encoding, contentType ContentType, charset Charset, body []byte) string {
+func (sm *SMime) prepareMessage(encoding Encoding, contentType ContentType, charset Charset, body []byte) string {
 	return fmt.Sprintf("Content-Transfer-Encoding: %v\r\nContent-Type: %v; charset=%v\r\n\r\n%v", encoding, contentType, charset, string(body))
 }
 

smime_test.go

@@ -0,0 +1,190 @@
+package mail
+
+import (
+	"bytes"
+	"encoding/base64"
+	"fmt"
+	"strings"
+	"testing"
+)
+
+// TestNewSMime tests the newSMime method
+func TestNewSMime(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("Error getting dummy certificate: %s", err)
+	}
+
+	sMime, err := newSMime(keyPair)
+	if err != nil {
+		t.Errorf("Error creating new SMime from keyPair: %s", err)
+	}
+
+	if sMime.privateKey != keyPair.PrivateKey {
+		t.Errorf("NewSMime() did not return the same private key")
+	}
+	if sMime.certificate != keyPair.Leaf {
+		t.Errorf("NewSMime() did not return the same leaf certificate")
+	}
+}
+
+// TestSign tests the sign method
+func TestSign(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("Error getting dummy certificate: %s", err)
+	}
+
+	sMime, err := newSMime(keyPair)
+	if err != nil {
+		t.Errorf("Error creating new SMime from keyPair: %s", err)
+	}
+
+	message := "This is a test message"
+	singedMessage, err := sMime.signMessage(message)
+	if err != nil {
+		t.Errorf("Error creating singed message: %s", err)
+	}
+
+	if *singedMessage == message {
+		t.Errorf("Sign() did not work")
+	}
+}
+
+// TestPrepareMessage tests the createMessage method
+func TestPrepareMessage(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("Error getting dummy certificate: %s", err)
+	}
+
+	sMime, err := newSMime(keyPair)
+	if err != nil {
+		t.Errorf("Error creating new SMime from keyPair: %s", err)
+	}
+
+	encoding := EncodingB64
+	contentType := TypeTextPlain
+	charset := CharsetUTF8
+	body := []byte("This is the body!")
+	result := sMime.prepareMessage(encoding, contentType, charset, body)
+
+	if !strings.Contains(result, encoding.String()) {
+		t.Errorf("createMessage() did not return the correct encoding")
+	}
+	if !strings.Contains(result, contentType.String()) {
+		t.Errorf("createMessage() did not return the correct contentType")
+	}
+	if !strings.Contains(result, string(body)) {
+		t.Errorf("createMessage() did not return the correct body")
+	}
+	if result != fmt.Sprintf("Content-Transfer-Encoding: %s\r\nContent-Type: %s; charset=%s\r\n\r\n%s", encoding, contentType, charset, string(body)) {
+		t.Errorf("createMessage() did not sucessfully create the message")
+	}
+}
+
+// TestEncodeToPEM tests the encodeToPEM method
+func TestEncodeToPEM(t *testing.T) {
+	message := []byte("This is a test message")
+
+	pemMessage, err := encodeToPEM(message)
+	if err != nil {
+		t.Errorf("Error encoding message: %s", err)
+	}
+
+	base64Encoded := base64.StdEncoding.EncodeToString(message)
+	if *pemMessage != base64Encoded {
+		t.Errorf("encodeToPEM() did not work")
+	}
+}
+
+// TestBytesFromLines tests the bytesFromLines method
+func TestBytesFromLines(t *testing.T) {
+	ls := lines{
+		{line: []byte("Hello"), endOfLine: []byte("\n")},
+		{line: []byte("World"), endOfLine: []byte("\n")},
+	}
+	expected := []byte("Hello\nWorld\n")
+
+	result := ls.bytesFromLines([]byte("\n"))
+	if !bytes.Equal(result, expected) {
+		t.Errorf("Expected %s, but got %s", expected, result)
+	}
+}
+
+// FuzzBytesFromLines tests the bytesFromLines method with fuzzing
+func FuzzBytesFromLines(f *testing.F) {
+	f.Add([]byte("Hello"), []byte("\n"))
+	f.Fuzz(func(t *testing.T, lineData, sep []byte) {
+		ls := lines{
+			{line: lineData, endOfLine: sep},
+		}
+		_ = ls.bytesFromLines(sep)
+	})
+}
+
+// TestParseLines tests the parseLines method
+func TestParseLines(t *testing.T) {
+	input := []byte("Hello\r\nWorld\nHello\rWorld")
+	expected := lines{
+		{line: []byte("Hello"), endOfLine: []byte("\r\n")},
+		{line: []byte("World"), endOfLine: []byte("\n")},
+		{line: []byte("Hello"), endOfLine: []byte("\r")},
+		{line: []byte("World"), endOfLine: []byte("")},
+	}
+
+	result := parseLines(input)
+	if len(result) != len(expected) {
+		t.Errorf("Expected %d lines, but got %d", len(expected), len(result))
+	}
+
+	for i := range result {
+		if !bytes.Equal(result[i].line, expected[i].line) || !bytes.Equal(result[i].endOfLine, expected[i].endOfLine) {
+			t.Errorf("Line %d mismatch. Expected line: %s, endOfLine: %s, got line: %s, endOfLine: %s",
+				i, expected[i].line, expected[i].endOfLine, result[i].line, result[i].endOfLine)
+		}
+	}
+}
+
+// FuzzParseLines tests the parseLines method with fuzzing
+func FuzzParseLines(f *testing.F) {
+	f.Add([]byte("Hello\nWorld\r\nAnother\rLine"))
+	f.Fuzz(func(t *testing.T, input []byte) {
+		_ = parseLines(input)
+	})
+}
+
+// TestSplitLine tests the splitLine method
+func TestSplitLine(t *testing.T) {
+	ls := lines{
+		{line: []byte("Hello\r\nWorld\r\nAnotherLine"), endOfLine: []byte("")},
+	}
+	expected := lines{
+		{line: []byte("Hello"), endOfLine: []byte("\r\n")},
+		{line: []byte("World"), endOfLine: []byte("\r\n")},
+		{line: []byte("AnotherLine"), endOfLine: []byte("")},
+	}
+
+	result := ls.splitLine([]byte("\r\n"))
+	if len(result) != len(expected) {
+		t.Errorf("Expected %d lines, but got %d", len(expected), len(result))
+	}
+
+	for i := range result {
+		if !bytes.Equal(result[i].line, expected[i].line) || !bytes.Equal(result[i].endOfLine, expected[i].endOfLine) {
+			t.Errorf("Line %d mismatch. Expected line: %s, endOfLine: %s, got line: %s, endOfLine: %s",
+				i, expected[i].line, expected[i].endOfLine, result[i].line, result[i].endOfLine)
+		}
+	}
+}
+
+// FuzzSplitLine tests the parseLsplitLineines method with fuzzing
+func FuzzSplitLine(f *testing.F) {
+	f.Add([]byte("Hello\r\nWorld"), []byte("\r\n"))
+	f.Fuzz(func(t *testing.T, input, sep []byte) {
+		ls := lines{
+			{line: input, endOfLine: []byte("")},
+		}
+		_ = ls.splitLine(sep)
+	})
+}

smtp/auth.go

@@ -13,6 +13,19 @@
 
 package smtp
 
+import "errors"
+
+var (
+	// ErrUnencrypted is an error indicating that the connection is not encrypted.
+	ErrUnencrypted = errors.New("unencrypted connection")
+	// ErrUnexpectedServerChallange is an error indicating that the server issued an unexpected challenge.
+	ErrUnexpectedServerChallange = errors.New("unexpected server challenge")
+	// ErrUnexpectedServerResponse is an error indicating that the server issued an unexpected response.
+	ErrUnexpectedServerResponse = errors.New("unexpected server response")
+	// ErrWrongHostname is an error indicating that the provided hostname does not match the expected value.
+	ErrWrongHostname = errors.New("wrong host name")
+)
+
 // Auth is implemented by an SMTP authentication mechanism.
 type Auth interface {
 	// Start begins an authentication with a server.

smtp/auth_login.go

@@ -1,11 +1,10 @@
-// SPDX-FileCopyrightText: Copyright (c) 2022-2023 The go-mail Authors
+// SPDX-FileCopyrightText: Copyright (c) 2022-2024 The go-mail Authors
 //
 // SPDX-License-Identifier: MIT
 
 package smtp
 
 import (
-	"errors"
 	"fmt"
 )
 
@@ -13,53 +12,35 @@ import (
 type loginAuth struct {
 	username, password string
 	host               string
+	respStep           uint8
 }
 
-const (
-	// LoginXUsernameChallenge represents the Username Challenge response sent by the SMTP server per the AUTH LOGIN
-	// extension.
-	//
-	// See: https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-xlogin/.
-	LoginXUsernameChallenge      = "Username:"
-	LoginXUsernameLowerChallenge = "username:"
-
-	// LoginXPasswordChallenge represents the Password Challenge response sent by the SMTP server per the AUTH LOGIN
-	// extension.
-	//
-	//	See: https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-xlogin/.
-	LoginXPasswordChallenge      = "Password:"
-	LoginXPasswordLowerChallenge = "password:"
-
-	// LoginXDraftUsernameChallenge represents the Username Challenge response sent by the SMTP server per the IETF
-	// draft AUTH LOGIN extension. It should be noted this extension is an expired draft which was never formally
-	// published and was deprecated in favor of the AUTH PLAIN extension.
-	//
-	// See: https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00.
-	LoginXDraftUsernameChallenge = "User Name\x00"
-
-	// LoginXDraftPasswordChallenge represents the Password Challenge response sent by the SMTP server per the IETF
-	// draft AUTH LOGIN extension. It should be noted this extension is an expired draft which was never formally
-	// published and was deprecated in favor of the AUTH PLAIN extension.
-	//
-	// See: https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00.
-	LoginXDraftPasswordChallenge = "Password\x00"
-)
-
 // LoginAuth returns an [Auth] that implements the LOGIN authentication
 // mechanism as it is used by MS Outlook. The Auth works similar to PLAIN
 // but instead of sending all in one response, the login is handled within
 // 3 steps:
-// - Sending AUTH LOGIN (server responds with "Username:")
+// - Sending AUTH LOGIN (server might responds with "Username:")
-// - Sending the username (server responds with "Password:")
+// - Sending the username (server might responds with "Password:")
 // - Sending the password (server authenticates)
+// This is the common approach as specified by Microsoft in their MS-XLOGIN spec.
+// See: https://msopenspecs.azureedge.net/files/MS-XLOGIN/%5bMS-XLOGIN%5d.pdf
+// Yet, there is also an old IETF draft for SMTP AUTH LOGIN that states for clients:
+// "The contents of both challenges SHOULD be ignored.".
+// See: https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00
+// Since there is no official standard RFC and we've seen different implementations
+// of this mechanism (sending "Username:", "Username", "username", "User name", etc.)
+// we follow the IETF-Draft and ignore any server challange to allow compatiblity
+// with most mail servers/providers.
 //
 // LoginAuth will only send the credentials if the connection is using TLS
 // or is connected to localhost. Otherwise authentication will fail with an
 // error, without sending the credentials.
 func LoginAuth(username, password, host string) Auth {
-	return &loginAuth{username, password, host}
+	return &loginAuth{username, password, host, 0}
 }
 
+// Start begins the SMTP authentication process by validating server's TLS status and hostname.
+// Returns "LOGIN" on success.
 func (a *loginAuth) Start(server *ServerInfo) (string, []byte, error) {
 	// Must have TLS, or else localhost server.
 	// Note: If TLS is not true, then we can't trust ANYTHING in ServerInfo.
@@ -67,23 +48,28 @@ func (a *loginAuth) Start(server *ServerInfo) (string, []byte, error) {
 	// That might just be the attacker saying
 	// "it's ok, you can trust me with your password."
 	if !server.TLS && !isLocalhost(server.Name) {
-		return "", nil, errors.New("unencrypted connection")
+		return "", nil, ErrUnencrypted
 	}
 	if server.Name != a.host {
-		return "", nil, errors.New("wrong host name")
+		return "", nil, ErrWrongHostname
 	}
+	a.respStep = 0
 	return "LOGIN", nil, nil
 }
 
+// Next processes responses from the server during the SMTP authentication exchange, sending the
+// username and password.
 func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
 	if more {
-		switch string(fromServer) {
+		switch a.respStep {
-		case LoginXUsernameChallenge, LoginXUsernameLowerChallenge, LoginXDraftUsernameChallenge:
+		case 0:
+			a.respStep++
 			return []byte(a.username), nil
-		case LoginXPasswordChallenge, LoginXPasswordLowerChallenge, LoginXDraftPasswordChallenge:
+		case 1:
+			a.respStep++
 			return []byte(a.password), nil
 		default:
-			return nil, fmt.Errorf("unexpected server response: %s", string(fromServer))
+			return nil, fmt.Errorf("%w: %s", ErrUnexpectedServerResponse, string(fromServer))
 		}
 	}
 	return nil, nil

smtp/auth_plain.go

@@ -13,10 +13,6 @@
 
 package smtp
 
-import (
-	"errors"
-)
-
 // plainAuth is the type that satisfies the Auth interface for the "SMTP PLAIN" auth
 type plainAuth struct {
 	identity, username, password string
@@ -42,10 +38,10 @@ func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
 	// That might just be the attacker saying
 	// "it's ok, you can trust me with your password."
 	if !server.TLS && !isLocalhost(server.Name) {
-		return "", nil, errors.New("unencrypted connection")
+		return "", nil, ErrUnencrypted
 	}
 	if server.Name != a.host {
-		return "", nil, errors.New("wrong host name")
+		return "", nil, ErrWrongHostname
 	}
 	resp := []byte(a.identity + "\x00" + a.username + "\x00" + a.password)
 	return "PLAIN", resp, nil
@@ -54,7 +50,7 @@ func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
 func (a *plainAuth) Next(_ []byte, more bool) ([]byte, error) {
 	if more {
 		// We've already sent everything.
-		return nil, errors.New("unexpected server challenge")
+		return nil, ErrUnexpectedServerChallange
 	}
 	return nil, nil
 }

smtp/auth_scram.go

@@ -0,0 +1,317 @@
+// SPDX-FileCopyrightText: Copyright (c) 2024 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+package smtp
+
+import (
+	"bytes"
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/sha1"
+	"crypto/sha256"
+	"crypto/tls"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"hash"
+	"io"
+	"strconv"
+	"strings"
+
+	"golang.org/x/crypto/pbkdf2"
+	"golang.org/x/text/secure/precis"
+)
+
+// scramAuth represents a SCRAM (Salted Challenge Response Authentication Mechanism) client and
+// satisfies the smtp.Auth interface.
+type scramAuth struct {
+	username, password, algorithm               string
+	firstBareMsg, nonce, saltedPwd, authMessage []byte
+	iterations                                  int
+	h                                           func() hash.Hash
+	isPlus                                      bool
+	tlsConnState                                *tls.ConnectionState
+	bindData                                    []byte
+}
+
+// ScramSHA1Auth creates and returns a new SCRAM-SHA-1 authentication mechanism with the given
+// username and password.
+func ScramSHA1Auth(username, password string) Auth {
+	return &scramAuth{
+		username:  username,
+		password:  password,
+		algorithm: "SCRAM-SHA-1",
+		h:         sha1.New,
+	}
+}
+
+// ScramSHA256Auth creates and returns a new SCRAM-SHA-256 authentication mechanism with the given
+// username and password.
+func ScramSHA256Auth(username, password string) Auth {
+	return &scramAuth{
+		username:  username,
+		password:  password,
+		algorithm: "SCRAM-SHA-256",
+		h:         sha256.New,
+	}
+}
+
+// ScramSHA1PlusAuth returns an Auth instance configured for SCRAM-SHA-1-PLUS authentication with
+// the provided username, password, and TLS connection state.
+func ScramSHA1PlusAuth(username, password string, tlsConnState *tls.ConnectionState) Auth {
+	return &scramAuth{
+		username:     username,
+		password:     password,
+		algorithm:    "SCRAM-SHA-1-PLUS",
+		h:            sha1.New,
+		isPlus:       true,
+		tlsConnState: tlsConnState,
+	}
+}
+
+// ScramSHA256PlusAuth returns an Auth instance configured for SCRAM-SHA-256-PLUS authentication with
+// the provided username, password, and TLS connection state.
+func ScramSHA256PlusAuth(username, password string, tlsConnState *tls.ConnectionState) Auth {
+	return &scramAuth{
+		username:     username,
+		password:     password,
+		algorithm:    "SCRAM-SHA-256-PLUS",
+		h:            sha256.New,
+		isPlus:       true,
+		tlsConnState: tlsConnState,
+	}
+}
+
+// Start initializes the SCRAM authentication process and returns the selected algorithm, nil data, and no error.
+func (a *scramAuth) Start(_ *ServerInfo) (string, []byte, error) {
+	return a.algorithm, nil, nil
+}
+
+// Next processes the server's challenge and returns the client's response for SCRAM authentication.
+func (a *scramAuth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		if len(fromServer) == 0 {
+			a.reset()
+			return a.initialClientMessage()
+		}
+		switch {
+		case bytes.HasPrefix(fromServer, []byte("r=")):
+			resp, err := a.handleServerFirstResponse(fromServer)
+			if err != nil {
+				a.reset()
+				return nil, err
+			}
+			return resp, nil
+		case bytes.HasPrefix(fromServer, []byte("v=")):
+			resp, err := a.handleServerValidationMessage(fromServer)
+			if err != nil {
+				a.reset()
+				return nil, err
+			}
+			return resp, nil
+		default:
+			a.reset()
+			return nil, fmt.Errorf("%w: %s", ErrUnexpectedServerResponse, string(fromServer))
+		}
+	}
+	return nil, nil
+}
+
+// reset clears all authentication-related properties in the scramAuth instance, effectively resetting its state.
+func (a *scramAuth) reset() {
+	a.nonce = nil
+	a.firstBareMsg = nil
+	a.saltedPwd = nil
+	a.authMessage = nil
+	a.iterations = 0
+}
+
+// initialClientMessage generates the initial message for SCRAM authentication, including a nonce and
+// optional channel binding.
+func (a *scramAuth) initialClientMessage() ([]byte, error) {
+	username, err := a.normalizeUsername()
+	if err != nil {
+		return nil, fmt.Errorf("username normalization failed: %w", err)
+	}
+
+	nonceBuffer := make([]byte, 24)
+	if _, err := io.ReadFull(rand.Reader, nonceBuffer); err != nil {
+		return nil, fmt.Errorf("unable to generate client secret: %w", err)
+	}
+	a.nonce = make([]byte, base64.StdEncoding.EncodedLen(len(nonceBuffer)))
+	base64.StdEncoding.Encode(a.nonce, nonceBuffer)
+
+	a.firstBareMsg = []byte("n=" + username + ",r=" + string(a.nonce))
+	returnBytes := []byte("n,," + string(a.firstBareMsg))
+
+	// SCRAM-SHA-X-PLUS auth requires channel binding
+	if a.isPlus {
+		if a.tlsConnState == nil {
+			return nil, errors.New("tls connection state is required for SCRAM-SHA-X-PLUS")
+		}
+		bindType := "tls-unique"
+		connState := a.tlsConnState
+		bindData := connState.TLSUnique
+
+		// crypto/tl: no tls-unique channel binding value for this tls connection, possibly due to missing
+		// extended master key support and/or resumed connection
+		// RFC9266:122 tls-unique not defined for tls 1.3 and later
+		if bindData == nil || connState.Version >= tls.VersionTLS13 {
+			bindType = "tls-exporter"
+			bindData, err = connState.ExportKeyingMaterial("EXPORTER-Channel-Binding", []byte{}, 32)
+			if err != nil {
+				return nil, fmt.Errorf("unable to export keying material: %w", err)
+			}
+		}
+		bindData = []byte("p=" + bindType + ",," + string(bindData))
+		a.bindData = make([]byte, base64.StdEncoding.EncodedLen(len(bindData)))
+		base64.StdEncoding.Encode(a.bindData, bindData)
+		returnBytes = []byte("p=" + bindType + ",," + string(a.firstBareMsg))
+	}
+
+	return returnBytes, nil
+}
+
+// handleServerFirstResponse processes the first response from the server in SCRAM authentication.
+func (a *scramAuth) handleServerFirstResponse(fromServer []byte) ([]byte, error) {
+	parts := bytes.Split(fromServer, []byte(","))
+	if len(parts) < 3 {
+		return nil, errors.New("not enough fields in the first server response")
+	}
+	if !bytes.HasPrefix(parts[0], []byte("r=")) {
+		return nil, errors.New("first part of the server response does not start with r=")
+	}
+	if !bytes.HasPrefix(parts[1], []byte("s=")) {
+		return nil, errors.New("second part of the server response does not start with s=")
+	}
+	if !bytes.HasPrefix(parts[2], []byte("i=")) {
+		return nil, errors.New("third part of the server response does not start with i=")
+	}
+
+	combinedNonce := parts[0][2:]
+	if len(a.nonce) == 0 || !bytes.HasPrefix(combinedNonce, a.nonce) {
+		return nil, errors.New("server nonce does not start with our nonce")
+	}
+	a.nonce = combinedNonce
+
+	encodedSalt := parts[1][2:]
+	salt := make([]byte, base64.StdEncoding.DecodedLen(len(encodedSalt)))
+	n, err := base64.StdEncoding.Decode(salt, encodedSalt)
+	if err != nil {
+		return nil, fmt.Errorf("invalid encoded salt: %w", err)
+	}
+	salt = salt[:n]
+
+	iterations, err := strconv.Atoi(string(parts[2][2:]))
+	if err != nil {
+		return nil, fmt.Errorf("invalid iterations: %w", err)
+	}
+	a.iterations = iterations
+
+	password, err := a.normalizeString(a.password)
+	if err != nil {
+		return nil, fmt.Errorf("unable to normalize password: %w", err)
+	}
+
+	a.saltedPwd = pbkdf2.Key([]byte(password), salt, a.iterations, a.h().Size(), a.h)
+
+	msgWithoutProof := []byte("c=biws,r=" + string(a.nonce))
+
+	// A PLUS authentication requires the channel binding data
+	if a.isPlus {
+		msgWithoutProof = []byte("c=" + string(a.bindData) + ",r=" + string(a.nonce))
+	}
+
+	a.authMessage = []byte(string(a.firstBareMsg) + "," + string(fromServer) + "," + string(msgWithoutProof))
+	clientProof := a.computeClientProof()
+
+	return []byte(string(msgWithoutProof) + ",p=" + string(clientProof)), nil
+}
+
+// handleServerValidationMessage verifies the server's signature during the SCRAM authentication process.
+func (a *scramAuth) handleServerValidationMessage(fromServer []byte) ([]byte, error) {
+	serverSignature := fromServer[2:]
+	computedServerSignature := a.computeServerSignature()
+
+	if !hmac.Equal(serverSignature, computedServerSignature) {
+		return nil, errors.New("invalid server signature")
+	}
+	return []byte(""), nil
+}
+
+// computeHMAC generates a Hash-based Message Authentication Code (HMAC) using the specified key and message.
+func (a *scramAuth) computeHMAC(key, msg []byte) []byte {
+	mac := hmac.New(a.h, key)
+	mac.Write(msg)
+	return mac.Sum(nil)
+}
+
+// computeHash generates a hash of the given key using the configured hashing algorithm.
+func (a *scramAuth) computeHash(key []byte) []byte {
+	hasher := a.h()
+	hasher.Write(key)
+	return hasher.Sum(nil)
+}
+
+// computeClientProof generates the client proof as part of the SCRAM authentication process.
+func (a *scramAuth) computeClientProof() []byte {
+	clientKey := a.computeHMAC(a.saltedPwd, []byte("Client Key"))
+	storedKey := a.computeHash(clientKey)
+	clientSignature := a.computeHMAC(storedKey[:], a.authMessage)
+	clientProof := make([]byte, len(clientSignature))
+	for i := 0; i < len(clientSignature); i++ {
+		clientProof[i] = clientKey[i] ^ clientSignature[i]
+	}
+	buf := make([]byte, base64.StdEncoding.EncodedLen(len(clientProof)))
+	base64.StdEncoding.Encode(buf, clientProof)
+	return buf
+}
+
+// computeServerSignature returns the computed base64-encoded server signature in the SCRAM
+// authentication process.
+func (a *scramAuth) computeServerSignature() []byte {
+	serverKey := a.computeHMAC(a.saltedPwd, []byte("Server Key"))
+	serverSignature := a.computeHMAC(serverKey, a.authMessage)
+	buf := make([]byte, base64.StdEncoding.EncodedLen(len(serverSignature)))
+	base64.StdEncoding.Encode(buf, serverSignature)
+	return buf
+}
+
+// normalizeUsername replaces special characters in the username for SCRAM authentication
+// and prepares it using the SASLprep profile as per RFC 8265, returning the normalized
+// username or an error.
+func (a *scramAuth) normalizeUsername() (string, error) {
+	// RFC 5802 section 5.1: the characters ',' or '=' in usernames are
+	// sent as '=2C' and '=3D' respectively.
+	replacer := strings.NewReplacer("=", "=3D", ",", "=2C")
+	username := replacer.Replace(a.username)
+	// RFC 5802 section 5.1: before sending the username to the server,
+	// the client SHOULD prepare the username using the "SASLprep"
+	// profile [RFC4013] of the "stringprep" algorithm [RFC3454]
+	// treating it as a query string (i.e., unassigned Unicode code
+	// points are allowed). If the preparation of the username fails or
+	// results in an empty string, the client SHOULD abort the
+	// authentication exchange.
+	//
+	// Since RFC 8265 obsoletes RFC 4013 we use it instead.
+	username, err := a.normalizeString(username)
+	if err != nil {
+		return "", fmt.Errorf("unable to normalize username: %w", err)
+	}
+	return username, nil
+}
+
+// normalizeString normalizes the input string according to the OpaqueString profile of the
+// precis framework. It returns the normalized string or an error if normalization fails or
+// results in an empty string.
+func (a *scramAuth) normalizeString(s string) (string, error) {
+	s, err := precis.OpaqueString.String(s)
+	if err != nil {
+		return "", fmt.Errorf("failled to normalize string: %w", err)
+	}
+	if s == "" {
+		return "", errors.New("normalized string is empty")
+	}
+	return s, nil
+}

smtp/smtp.go

@@ -30,34 +30,70 @@ import (
 	"net/textproto"
 	"os"
 	"strings"
+	"sync"
+	"time"
 
 	"github.com/wneessen/go-mail/log"
 )
 
+var (
+
+	// ErrNonTLSConnection is returned when an attempt is made to retrieve TLS state on a non-TLS connection.
+	ErrNonTLSConnection = errors.New("connection is not using TLS")
+
+	// ErrNoConnection is returned when attempting to perform an operation that requires an established
+	// connection but none exists.
+	ErrNoConnection = errors.New("connection is not established")
+)
+
 // A Client represents a client connection to an SMTP server.
 type Client struct {
-	// Text is the textproto.Conn used by the Client. It is exported to allow for
+	// Text is the textproto.Conn used by the Client. It is exported to allow for clients to add extensions.
-	// clients to add extensions.
 	Text *textproto.Conn
-	// keep a reference to the connection so it can be used to create a TLS
+
-	// connection later
+	// auth supported auth mechanisms
+	auth []string
+
+	// keep a reference to the connection so it can be used to create a TLS connection later
 	conn net.Conn
-	// whether the Client is using TLS
+
-	tls        bool
+	// debug logging is enabled
-	serverName string
+	debug bool
-	// map of supported extensions
+
+	// didHello indicates whether we've said HELO/EHLO
+	didHello bool
+
+	// dsnmrtype defines the mail return option in case DSN is enabled
+	dsnmrtype string
+
+	// dsnrntype defines the recipient notify option in case DSN is enabled
+	dsnrntype string
+
+	// ext is a map of supported extensions
 	ext map[string]string
-	// supported auth mechanisms
+
-	auth       []string
+	// helloError is the error from the hello
-	localName  string // the name to use in HELO/EHLO
+	helloError error
-	didHello   bool   // whether we've said HELO/EHLO
+
-	helloError error  // the error from the hello
+	// isConnected indicates if the Client has an active connection
-	// debug logging
+	isConnected bool
-	debug  bool       // debug logging is enabled
+
-	logger log.Logger // logger will be used for debug logging
+	// localName is the name to use in HELO/EHLO
-	// DSN support
+	localName string // the name to use in HELO/EHLO
-	dsnmrtype string // dsnmrtype defines the mail return option in case DSN is enabled
+
-	dsnrntype string // dsnrntype defines the recipient notify option in case DSN is enabled
+	// logger will be used for debug logging
+	logger log.Logger
+
+	// mutex is used to synchronize access to shared resources, ensuring that only one goroutine can access
+	// the resource at a time.
+	mutex sync.RWMutex
+
+	// tls indicates whether the Client is using TLS
+	tls bool
+
+	// serverName denotes the name of the server to which the application will connect. Used for
+	// identification and routing.
+	serverName string
 }
 
 // Dial returns a new [Client] connected to an SMTP server at addr.
@@ -88,13 +124,18 @@ func NewClient(conn net.Conn, host string) (*Client, error) {
 	}
 	c := &Client{Text: text, conn: conn, serverName: host, localName: "localhost"}
 	_, c.tls = conn.(*tls.Conn)
+	c.isConnected = true
 
 	return c, nil
 }
 
 // Close closes the connection.
 func (c *Client) Close() error {
-	return c.Text.Close()
+	c.mutex.Lock()
+	err := c.Text.Close()
+	c.isConnected = false
+	c.mutex.Unlock()
+	return err
 }
 
 // hello runs a hello exchange if needed.
@@ -121,28 +162,39 @@ func (c *Client) Hello(localName string) error {
 	if c.didHello {
 		return errors.New("smtp: Hello called after other methods")
 	}
+
+	c.mutex.Lock()
 	c.localName = localName
+	c.mutex.Unlock()
+
 	return c.hello()
 }
 
 // cmd is a convenience function that sends a command and returns the response
 func (c *Client) cmd(expectCode int, format string, args ...interface{}) (int, string, error) {
+	c.mutex.Lock()
+
 	c.debugLog(log.DirClientToServer, format, args...)
 	id, err := c.Text.Cmd(format, args...)
 	if err != nil {
+		c.mutex.Unlock()
 		return 0, "", err
 	}
 	c.Text.StartResponse(id)
-	defer c.Text.EndResponse(id)
 	code, msg, err := c.Text.ReadResponse(expectCode)
 	c.debugLog(log.DirServerToClient, "%d %s", code, msg)
+	c.Text.EndResponse(id)
+	c.mutex.Unlock()
 	return code, msg, err
 }
 
 // helo sends the HELO greeting to the server. It should be used only when the
 // server does not support ehlo.
 func (c *Client) helo() error {
+	c.mutex.Lock()
 	c.ext = nil
+	c.mutex.Unlock()
+
 	_, _, err := c.cmd(250, "HELO %s", c.localName)
 	return err
 }
@@ -157,9 +209,13 @@ func (c *Client) StartTLS(config *tls.Config) error {
 	if err != nil {
 		return err
 	}
+
+	c.mutex.Lock()
 	c.conn = tls.Client(c.conn, config)
 	c.Text = textproto.NewConn(c.conn)
 	c.tls = true
+	c.mutex.Unlock()
+
 	return c.ehlo()
 }
 
@@ -167,11 +223,15 @@ func (c *Client) StartTLS(config *tls.Config) error {
 // The return values are their zero values if [Client.StartTLS] did
 // not succeed.
 func (c *Client) TLSConnectionState() (state tls.ConnectionState, ok bool) {
+	c.mutex.RLock()
+	defer c.mutex.RUnlock()
+
 	tc, ok := c.conn.(*tls.Conn)
 	if !ok {
 		return
 	}
-	return tc.ConnectionState(), true
+	state, ok = tc.ConnectionState(), true
+	return
 }
 
 // Verify checks the validity of an email address on the server.
@@ -257,6 +317,8 @@ func (c *Client) Mail(from string) error {
 		return err
 	}
 	cmdStr := "MAIL FROM:<%s>"
+
+	c.mutex.RLock()
 	if c.ext != nil {
 		if _, ok := c.ext["8BITMIME"]; ok {
 			cmdStr += " BODY=8BITMIME"
@@ -269,6 +331,8 @@ func (c *Client) Mail(from string) error {
 			cmdStr += fmt.Sprintf(" RET=%s", c.dsnmrtype)
 		}
 	}
+	c.mutex.RUnlock()
+
 	_, _, err := c.cmd(250, cmdStr, from)
 	return err
 }
@@ -280,7 +344,11 @@ func (c *Client) Rcpt(to string) error {
 	if err := validateLine(to); err != nil {
 		return err
 	}
+
+	c.mutex.RLock()
 	_, ok := c.ext["DSN"]
+	c.mutex.RUnlock()
+
 	if ok && c.dsnrntype != "" {
 		_, _, err := c.cmd(25, "RCPT TO:<%s> NOTIFY=%s", to, c.dsnrntype)
 		return err
@@ -294,12 +362,23 @@ type dataCloser struct {
 	io.WriteCloser
 }
 
+// Close releases the lock, closes the WriteCloser, waits for a response, and then returns any error encountered.
 func (d *dataCloser) Close() error {
+	d.c.mutex.Lock()
 	_ = d.WriteCloser.Close()
 	_, _, err := d.c.Text.ReadResponse(250)
+	d.c.mutex.Unlock()
 	return err
 }
 
+// Write writes data to the underlying WriteCloser while ensuring thread-safety by locking and unlocking a mutex.
+func (d *dataCloser) Write(p []byte) (n int, err error) {
+	d.c.mutex.Lock()
+	n, err = d.WriteCloser.Write(p)
+	d.c.mutex.Unlock()
+	return
+}
+
 // Data issues a DATA command to the server and returns a writer that
 // can be used to write the mail headers and body. The caller should
 // close the writer before calling any more methods on c. A call to
@@ -309,7 +388,14 @@ func (c *Client) Data() (io.WriteCloser, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &dataCloser{c, c.Text.DotWriter()}, nil
+	datacloser := &dataCloser{}
+
+	c.mutex.Lock()
+	datacloser.c = c
+	datacloser.WriteCloser = c.Text.DotWriter()
+	c.mutex.Unlock()
+
+	return datacloser, nil
 }
 
 var testHookStartTLS func(*tls.Config) // nil, except for tests
@@ -405,7 +491,10 @@ func (c *Client) Extension(ext string) (bool, string) {
 		return false, ""
 	}
 	ext = strings.ToUpper(ext)
+
+	c.mutex.RLock()
 	param, ok := c.ext[ext]
+	c.mutex.RUnlock()
 	return ok, param
 }
 
@@ -438,7 +527,12 @@ func (c *Client) Quit() error {
 	if err != nil {
 		return err
 	}
-	return c.Text.Close()
+	c.mutex.Lock()
+	err = c.Text.Close()
+	c.isConnected = false
+	c.mutex.Unlock()
+
+	return err
 }
 
 // SetDebugLog enables the debug logging for incoming and outgoing SMTP messages
@@ -472,6 +566,44 @@ func (c *Client) SetDSNRcptNotifyOption(d string) {
 	c.dsnrntype = d
 }
 
+// HasConnection checks if the client has an active connection.
+// Returns true if the `conn` field is not nil, indicating an active connection.
+func (c *Client) HasConnection() bool {
+	c.mutex.RLock()
+	isConn := c.isConnected
+	c.mutex.RUnlock()
+	return isConn
+}
+
+// UpdateDeadline sets a new deadline on the SMTP connection with the specified timeout duration.
+func (c *Client) UpdateDeadline(timeout time.Duration) error {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+	if err := c.conn.SetDeadline(time.Now().Add(timeout)); err != nil {
+		return fmt.Errorf("smtp: failed to update deadline: %w", err)
+	}
+	return nil
+}
+
+// GetTLSConnectionState retrieves the TLS connection state of the client's current connection.
+// Returns an error if the connection is not using TLS or if the connection is not established.
+func (c *Client) GetTLSConnectionState() (*tls.ConnectionState, error) {
+	c.mutex.RLock()
+	defer c.mutex.RUnlock()
+
+	if !c.isConnected {
+		return nil, ErrNoConnection
+	}
+	if !c.tls {
+		return nil, ErrNonTLSConnection
+	}
+	if conn, ok := c.conn.(*tls.Conn); ok {
+		cstate := conn.ConnectionState()
+		return &cstate, nil
+	}
+	return nil, errors.New("unable to retrieve TLS connection state")
+}
+
 // debugLog checks if the debug flag is set and if so logs the provided message to
 // the log.Logger interface
 func (c *Client) debugLog(d log.Direction, f string, a ...interface{}) {

smtp/smtp_ehlo.go

@@ -25,6 +25,9 @@ func (c *Client) ehlo() error {
 	if err != nil {
 		return err
 	}
+
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
 	ext := make(map[string]string)
 	extList := strings.Split(msg, "\n")
 	if len(extList) > 1 {

smtp/smtp_ehlo_117.go

@@ -22,12 +22,15 @@ import "strings"
 // should be the preferred greeting for servers that support it.
 //
 // Backport of: https://github.com/golang/go/commit/4d8db00641cc9ff4f44de7df9b8c4f4a4f9416ee#diff-4f6f6bdb9891d4dd271f9f31430420a2e44018fe4ee539576faf458bebb3cee4
-// to guarantee backwards compatibility with Go 1.16/1.17:w
+// to guarantee backwards compatibility with Go 1.16/1.17
 func (c *Client) ehlo() error {
 	_, msg, err := c.cmd(250, "EHLO %s", c.localName)
 	if err != nil {
 		return err
 	}
+
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
 	ext := make(map[string]string)
 	extList := strings.Split(msg, "\n")
 	if len(extList) > 1 {

tls.go

@@ -4,25 +4,32 @@
 
 package mail
 
-// TLSPolicy type describes a int alias for the different TLS policies we allow
+// TLSPolicy is a type wrapper for an int type and describes the different TLS policies we allow.
 type TLSPolicy int
 
 const (
 	// TLSMandatory requires that the connection to the server is
 	// encrypting using STARTTLS. If the server does not support STARTTLS
-	// the connection will be terminated with an error
+	// the connection will be terminated with an error.
 	TLSMandatory TLSPolicy = iota
 
 	// TLSOpportunistic tries to establish an encrypted connection via the
 	// STARTTLS protocol. If the server does not support this, it will fall
-	// back to non-encrypted plaintext transmission
+	// back to non-encrypted plaintext transmission.
 	TLSOpportunistic
 
-	// NoTLS forces the transaction to be not encrypted
+	// NoTLS forces the transaction to be not encrypted.
 	NoTLS
 )
 
-// String is a standard method to convert a TLSPolicy into a printable format
+// String satisfies the fmt.Stringer interface for the TLSPolicy type.
+//
+// This function returns a string representation of the TLSPolicy. It matches the policy
+// value to predefined constants and returns the corresponding string. If the policy does
+// not match any known values, it returns "UnknownPolicy".
+//
+// Returns:
+//   - A string representing the TLSPolicy.
 func (p TLSPolicy) String() string {
 	switch p {
 	case TLSMandatory:

util_test.go

@@ -10,6 +10,7 @@ const (
 	keyFilePath  = "dummy-child-key.pem"
 )
 
+// getDummyCertificate loads a certificate and a private key form local disk for testing purposes
 func getDummyCertificate() (*tls.Certificate, error) {
 	keyPair, err := tls.LoadX509KeyPair(certFilePath, keyFilePath)
 	if err != nil {