Merge pull request #324 from wneessen/better-godoc

Revision of the GoDoc documentation

README.md

@@ -52,7 +52,7 @@ Here are some highlights of go-mail's featureset:
 * [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

auth.go

@@ -6,69 +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 represents the SCRAM-SHA-1 SMTP authentication mechanism
+	// 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 represents the "SCRAM-SHA-1-PLUS" authentication mechanism for SMTP.
+	// 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 represents the SCRAM-SHA-256 authentication mechanism for SMTP.
+	// 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 represents the "SCRAM-SHA-256-PLUS" SMTP AUTH type.
+	// 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 should be used if the target server does not support the "SCRAM-SHA-1" schema
+	// 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 should be used if the target server does not support the "SCRAM-SHA-1-PLUS" schema
+	// 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 should be used if the target server does not support the "SCRAM-SHA-256" schema
+	// 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 should be used if the target server does not support the "SCRAM-SHA-256-PLUS" schema
+	// 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

@@ -9,7 +9,23 @@ package mail
 
 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 err := c.checkConn(); err != nil {
 		return &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}

client_120.go

@@ -11,7 +11,21 @@ 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 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)}

client_test.go

@@ -483,20 +483,20 @@ func TestWithDSN(t *testing.T) {
 		t.Errorf("failed to create new client: %s", err)
 		return
 	}
-	if !c.dsn {
+	if !c.requestDSN {
-		t.Errorf("WithDSN failed. c.dsn expected to be: %t, got: %t", true, c.dsn)
+		t.Errorf("WithDSN failed. c.requestDSN expected to be: %t, got: %t", true, c.requestDSN)
 	}
-	if c.dsnmrtype != DSNMailReturnFull {
+	if c.dsnReturnType != DSNMailReturnFull {
-		t.Errorf("WithDSN failed. c.dsnmrtype expected to be: %s, got: %s", DSNMailReturnFull,
+		t.Errorf("WithDSN failed. c.dsnReturnType expected to be: %s, got: %s", DSNMailReturnFull,
-			c.dsnmrtype)
+			c.dsnReturnType)
 	}
-	if c.dsnrntype[0] != string(DSNRcptNotifyFailure) {
+	if c.dsnRcptNotifyType[0] != string(DSNRcptNotifyFailure) {
-		t.Errorf("WithDSN failed. c.dsnrntype[0] expected to be: %s, got: %s", DSNRcptNotifyFailure,
+		t.Errorf("WithDSN failed. c.dsnRcptNotifyType[0] expected to be: %s, got: %s", DSNRcptNotifyFailure,
-			c.dsnrntype[0])
+			c.dsnRcptNotifyType[0])
 	}
-	if c.dsnrntype[1] != string(DSNRcptNotifySuccess) {
+	if c.dsnRcptNotifyType[1] != string(DSNRcptNotifySuccess) {
-		t.Errorf("WithDSN failed. c.dsnrntype[1] expected to be: %s, got: %s", DSNRcptNotifySuccess,
+		t.Errorf("WithDSN failed. c.dsnRcptNotifyType[1] expected to be: %s, got: %s", DSNRcptNotifySuccess,
-			c.dsnrntype[1])
+			c.dsnRcptNotifyType[1])
 	}
 }
 
@@ -519,8 +519,8 @@ func TestWithDSNMailReturnType(t *testing.T) {
 				t.Errorf("failed to create new client: %s", err)
 				return
 			}
-			if string(c.dsnmrtype) != tt.want {
+			if string(c.dsnReturnType) != tt.want {
-				t.Errorf("WithDSNMailReturnType failed. Expected %s, got: %s", tt.want, string(c.dsnmrtype))
+				t.Errorf("WithDSNMailReturnType failed. Expected %s, got: %s", tt.want, string(c.dsnReturnType))
 			}
 		})
 	}
@@ -547,11 +547,11 @@ func TestWithDSNRcptNotifyType(t *testing.T) {
 				t.Errorf("failed to create new client: %s", err)
 				return
 			}
-			if len(c.dsnrntype) <= 0 && !tt.sf {
+			if len(c.dsnRcptNotifyType) <= 0 && !tt.sf {
 				t.Errorf("WithDSNRcptNotifyType failed. Expected at least one DSNRNType but got none")
 			}
-			if !tt.sf && c.dsnrntype[0] != tt.want {
+			if !tt.sf && c.dsnRcptNotifyType[0] != tt.want {
-				t.Errorf("WithDSNRcptNotifyType failed. Expected %s, got: %s", tt.want, c.dsnrntype[0])
+				t.Errorf("WithDSNRcptNotifyType failed. Expected %s, got: %s", tt.want, c.dsnRcptNotifyType[0])
 			}
 		})
 	}
@@ -602,6 +602,10 @@ func TestSetSMTPAuthCustom(t *testing.T) {
 			if c.smtpAuth == nil {
 				t.Errorf("failed to set custom SMTP auth method. SMTP Auth method is empty")
 			}
+			if c.smtpAuthType != SMTPAuthCustom {
+				t.Errorf("failed to set custom SMTP auth method. SMTP Auth type is not custom: %s",
+					c.smtpAuthType)
+			}
 			p, _, err := c.smtpAuth.Start(&si)
 			if err != nil {
 				t.Errorf("SMTP Auth Start() method returned error: %s", err)
@@ -613,6 +617,32 @@ func TestSetSMTPAuthCustom(t *testing.T) {
 	}
 }
 
+// TestClient_Close_double tests if a close on an already closed connection causes an error.
+func TestClient_Close_double(t *testing.T) {
+	c, err := getTestConnection(true)
+	if err != nil {
+		t.Skipf("failed to create test client: %s. Skipping tests", err)
+	}
+	ctx := context.Background()
+	if err = c.DialWithContext(ctx); err != nil {
+		t.Errorf("failed to dial with context: %s", err)
+		return
+	}
+	if c.smtpClient == nil {
+		t.Errorf("DialWithContext didn't fail but no SMTP client found.")
+		return
+	}
+	if !c.smtpClient.HasConnection() {
+		t.Errorf("DialWithContext didn't fail but no connection found.")
+	}
+	if err = c.Close(); err != nil {
+		t.Errorf("failed to close connection: %s", err)
+	}
+	if err = c.Close(); err != nil {
+		t.Errorf("failed 2nd close connection: %s", err)
+	}
+}
+
 // TestClient_DialWithContext tests the DialWithContext method for the Client object
 func TestClient_DialWithContext(t *testing.T) {
 	c, err := getTestConnection(true)
@@ -2387,7 +2417,6 @@ func TestXOAuth2OK_faker(t *testing.T) {
 		"250 8BITMIME",
 		"250 OK",
 		"235 2.7.0 Accepted",
-		"250 OK",
 		"221 OK",
 	}
 	var wrote strings.Builder
@@ -2408,10 +2437,10 @@ func TestXOAuth2OK_faker(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unable to create new client: %v", err)
 	}
-	if err := c.DialWithContext(context.Background()); err != nil {
+	if err = c.DialWithContext(context.Background()); err != nil {
 		t.Fatalf("unexpected dial error: %v", err)
 	}
-	if err := c.Close(); err != nil {
+	if err = c.Close(); err != nil {
 		t.Fatalf("disconnect from test server failed: %v", err)
 	}
 	if !strings.Contains(wrote.String(), "AUTH XOAUTH2 dXNlcj11c2VyAWF1dGg9QmVhcmVyIHRva2VuAQE=\r\n") {
@@ -2426,7 +2455,6 @@ func TestXOAuth2Unsupported_faker(t *testing.T) {
 		"250-AUTH LOGIN PLAIN",
 		"250 8BITMIME",
 		"250 OK",
-		"250 OK",
 		"221 OK",
 	}
 	var wrote strings.Builder
@@ -2445,18 +2473,18 @@ func TestXOAuth2Unsupported_faker(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unable to create new client: %v", err)
 	}
-	if err := c.DialWithContext(context.Background()); err == nil {
+	if err = c.DialWithContext(context.Background()); err == nil {
 		t.Fatal("expected dial error got nil")
 	} else {
 		if !errors.Is(err, ErrXOauth2AuthNotSupported) {
 			t.Fatalf("expected %v; got %v", ErrXOauth2AuthNotSupported, err)
 		}
 	}
-	if err := c.Close(); err != nil {
+	if err = c.Close(); err != nil {
 		t.Fatalf("disconnect from test server failed: %v", err)
 	}
 	client := strings.Split(wrote.String(), "\r\n")
-	if len(client) != 5 {
+	if len(client) != 4 {
 		t.Fatalf("unexpected number of client requests got %d; want 5", len(client))
 	}
 	if !strings.HasPrefix(client[0], "EHLO") {
@@ -2465,10 +2493,7 @@ func TestXOAuth2Unsupported_faker(t *testing.T) {
 	if client[1] != "NOOP" {
 		t.Fatalf("expected NOOP, got %q", client[1])
 	}
-	if client[2] != "NOOP" {
+	if client[2] != "QUIT" {
-		t.Fatalf("expected NOOP, got %q", client[2])
-	}
-	if client[3] != "QUIT" {
 		t.Fatalf("expected QUIT, got %q", client[3])
 	}
 }

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
+// agent string.
 const VERSION = "0.4.4"

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

@@ -4,171 +4,218 @@
 
 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.
+	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"
 )
 
-// List of MIMETypes
 const (
+	// MIMEAlternative MIMEType represents a MIME multipart/alternative type, used for emails with multiple versions.
 	MIMEAlternative MIMEType = "alternative"
-	MIMEMixed       MIMEType = "mixed"
+
-	MIMERelated     MIMEType = "related"
+	// 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"
 )
 
-// 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)
 }

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

@@ -7,6 +7,6 @@ module github.com/wneessen/go-mail
 go 1.16
 
 require (
-	golang.org/x/crypto v0.27.0
+	golang.org/x/crypto v0.28.0
-	golang.org/x/text v0.18.0
+	golang.org/x/text v0.19.0
 )

go.sum

@@ -5,8 +5,8 @@ golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5y
 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 v0.27.0 h1:GXm2NjJrPaiv/h1tb2UH8QfgC/hOf/+z0p6PT8o1w7A=
+golang.org/x/crypto v0.28.0 h1:GBDwsMXVQi34v5CCYUm2jkJvu4cbtru2U4TN2PSyQnw=
-golang.org/x/crypto v0.27.0/go.mod h1:1Xngt8kV6Dvbssa53Ziq6Eqn0HqbZi5Z6R0ZpwQzt70=
+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=
@@ -37,7 +37,7 @@ 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 v0.25.0/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=
@@ -46,7 +46,7 @@ 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 v0.24.0/go.mod h1:lOBK/LVxemqiMij05LGJ0tzNr8xlmwBRJ81PX6wVLH8=
+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=
@@ -55,8 +55,8 @@ 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 v0.18.0 h1:XvMDiNzPAl0jr17s6W9lcaIhGUfUORdGCNsuLmPG224=
+golang.org/x/text v0.19.0 h1:kTxAhCbGbxhK0IwgSKiMO5awPoDQ0RpfiVYBfK860YM=
-golang.org/x/text v0.18.0/go.mod h1:BuEKDfySbSR4drPmRPG/7iBdf8hvFMuRexcpahXilzY=
+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=

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_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()
@@ -136,7 +176,13 @@ 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 {
@@ -148,14 +194,32 @@ 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 != "" {
@@ -175,7 +239,10 @@ 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()
@@ -183,7 +250,17 @@ 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
@@ -242,12 +319,29 @@ 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() == "" {
@@ -272,7 +366,14 @@ func (mw *msgWriter) writePart(part *Part, charset Charset) {
 	mw.writeBody(part.writeFunc, part.encoding)
 }
 
-// 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
@@ -282,7 +383,16 @@ 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
@@ -317,7 +427,17 @@ 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).
 func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encoding Encoding) {
 	var writer io.Writer
 	var encodedWriter io.WriteCloser

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
@@ -22,7 +26,14 @@ type Part struct {
 	writeFunc   func(io.Writer) (int64, error)
 }
 
-// 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 {
@@ -31,83 +42,172 @@ 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
 }
 
-// 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
 }
 
-// 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

@@ -14,14 +14,33 @@ import (
 const cr = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
 
 // 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 93 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 (7) needed to represent a letter index.
-	letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
+	letterIdxBits = 7
-	letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
+	// 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)

random_119.go

@@ -12,7 +12,17 @@ import (
 	"time"
 )
 
-// randNum returns a random number with a maximum value of length
+// randNum returns a random number with a maximum value of maxval.
+//
+// This function generates a random integer between 0 and maxval (exclusive). It seeds the
+// random number generator with the current time in nanoseconds to ensure different results
+// each time the function is called.
+//
+// Parameters:
+//   - maxval: The upper bound for the random number generation (exclusive).
+//
+// Returns:
+//   - A random integer between 0 and maxval. If maxval is less than or equal to 0, it returns 0.
 func randNum(maxval int) int {
 	if maxval <= 0 {
 		return 0

random_121.go

@@ -11,7 +11,17 @@ import (
 	"math/rand"
 )
 
-// randNum returns a random number with a maximum value of length
+// randNum returns a random number with a maximum value of maxval.
+//
+// This function generates a random integer between 0 and maxval (exclusive). If maxval is less
+// than or equal to 0, it returns 0. The random number generator uses the default seed provided
+// by the rand package.
+//
+// Parameters:
+//   - maxval: The upper bound for the random number generation (exclusive).
+//
+// Returns:
+//   - A random integer between 0 and maxval. If maxval is less than or equal to 0, it returns 0.
 func randNum(maxval int) int {
 	if maxval <= 0 {
 		return 0

random_122.go

@@ -12,8 +12,16 @@ import (
 )
 
 // randNum returns a random number with a maximum value of maxval.
-// go-mail compiled with Go 1.22+ will make use of the novel math/rand/v2 interface
+//
-// Older versions of Go will use math/rand
+// This function generates a random integer between 0 and maxval (exclusive). It utilizes
+// the math/rand/v2 interface for Go 1.22+ and will default to math/rand for older Go versions.
+// If maxval is less than or equal to 0, it returns 0.
+//
+// Parameters:
+//   - maxval: The upper bound for the random number generation (exclusive).
+//
+// Returns:
+//   - A random integer between 0 and maxval. If maxval is less than or equal to 0, it returns 0.
 func randNum(maxval int) int {
 	if maxval <= 0 {
 		return 0

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

@@ -54,7 +54,11 @@ 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 {
 	affectedMsg *Msg
 	errlist     []error
@@ -66,7 +70,16 @@ type SendError struct {
 // 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"
@@ -101,7 +114,17 @@ func (e *SendError) Error() string {
 	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 {
@@ -110,7 +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
@@ -118,8 +147,13 @@ func (e *SendError) IsTemp() bool {
 	return e.isTemp
 }
 
-// MessageID returns the message ID of the affected Msg that caused the error
+// MessageID returns the message ID of the affected Msg that caused the error.
-// If no message ID was set for the Msg, an empty string will be returned
+//
+// 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 ""
@@ -127,7 +161,13 @@ func (e *SendError) MessageID() string {
 	return e.affectedMsg.GetMessageID()
 }
 
-// Msg returns the pointer to the affected message that caused the error
+// 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
@@ -135,7 +175,14 @@ func (e *SendError) Msg() *Msg {
 	return e.affectedMsg
 }
 
-// String implements the Stringer interface for the SendErrReason
+// 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:
@@ -164,8 +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'
 }

smtp/smtp.go

@@ -36,7 +36,15 @@ import (
 	"github.com/wneessen/go-mail/log"
 )
 
-var ErrNonTLSConnection = errors.New("connection is not using TLS")
+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 {
@@ -67,6 +75,9 @@ type Client struct {
 	// helloError is the error from the hello
 	helloError error
 
+	// isConnected indicates if the Client has an active connection
+	isConnected bool
+
 	// localName is the name to use in HELO/EHLO
 	localName string // the name to use in HELO/EHLO
 
@@ -113,6 +124,7 @@ 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
 }
@@ -121,6 +133,7 @@ func NewClient(conn net.Conn, host string) (*Client, error) {
 func (c *Client) Close() error {
 	c.mutex.Lock()
 	err := c.Text.Close()
+	c.isConnected = false
 	c.mutex.Unlock()
 	return err
 }
@@ -516,6 +529,7 @@ func (c *Client) Quit() error {
 	}
 	c.mutex.Lock()
 	err = c.Text.Close()
+	c.isConnected = false
 	c.mutex.Unlock()
 
 	return err
@@ -555,15 +569,19 @@ func (c *Client) SetDSNRcptNotifyOption(d string) {
 // 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 {
-	return c.conn != nil
+	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)
 	}
-	c.mutex.Unlock()
 	return nil
 }
 
@@ -573,17 +591,17 @@ 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 c.conn == nil {
-		return nil, errors.New("smtp: connection is not established")
-	}
 	if conn, ok := c.conn.(*tls.Conn); ok {
 		cstate := conn.ConnectionState()
 		return &cstate, nil
 	}
-	return nil, errors.New("smtp: connection is not a TLS connection")
+	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

smtp/smtp_test.go

@@ -1640,6 +1640,357 @@ func TestTLSConnState(t *testing.T) {
 	<-serverDone
 }
 
+func TestClient_GetTLSConnectionState(t *testing.T) {
+	ln := newLocalListener(t)
+	defer func() {
+		_ = ln.Close()
+	}()
+	clientDone := make(chan bool)
+	serverDone := make(chan bool)
+	go func() {
+		defer close(serverDone)
+		c, err := ln.Accept()
+		if err != nil {
+			t.Errorf("Server accept: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Close()
+		}()
+		if err := serverHandle(c, t); err != nil {
+			t.Errorf("server error: %v", err)
+		}
+	}()
+	go func() {
+		defer close(clientDone)
+		c, err := Dial(ln.Addr().String())
+		if err != nil {
+			t.Errorf("Client dial: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Quit()
+		}()
+		cfg := &tls.Config{ServerName: "example.com"}
+		testHookStartTLS(cfg) // set the RootCAs
+		if err := c.StartTLS(cfg); err != nil {
+			t.Errorf("StartTLS: %v", err)
+			return
+		}
+		cs, err := c.GetTLSConnectionState()
+		if err != nil {
+			t.Errorf("failed to get TLSConnectionState: %s", err)
+			return
+		}
+		if cs.Version == 0 || !cs.HandshakeComplete {
+			t.Errorf("ConnectionState = %#v; expect non-zero Version and HandshakeComplete", cs)
+		}
+	}()
+	<-clientDone
+	<-serverDone
+}
+
+func TestClient_GetTLSConnectionState_noTLS(t *testing.T) {
+	ln := newLocalListener(t)
+	defer func() {
+		_ = ln.Close()
+	}()
+	clientDone := make(chan bool)
+	serverDone := make(chan bool)
+	go func() {
+		defer close(serverDone)
+		c, err := ln.Accept()
+		if err != nil {
+			t.Errorf("Server accept: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Close()
+		}()
+		if err := serverHandle(c, t); err != nil {
+			t.Errorf("server error: %v", err)
+		}
+	}()
+	go func() {
+		defer close(clientDone)
+		c, err := Dial(ln.Addr().String())
+		if err != nil {
+			t.Errorf("Client dial: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Quit()
+		}()
+		_, err = c.GetTLSConnectionState()
+		if err == nil {
+			t.Error("GetTLSConnectionState: expected error; got nil")
+			return
+		}
+	}()
+	<-clientDone
+	<-serverDone
+}
+
+func TestClient_GetTLSConnectionState_noConn(t *testing.T) {
+	ln := newLocalListener(t)
+	defer func() {
+		_ = ln.Close()
+	}()
+	clientDone := make(chan bool)
+	serverDone := make(chan bool)
+	go func() {
+		defer close(serverDone)
+		c, err := ln.Accept()
+		if err != nil {
+			t.Errorf("Server accept: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Close()
+		}()
+		if err := serverHandle(c, t); err != nil {
+			t.Errorf("server error: %v", err)
+		}
+	}()
+	go func() {
+		defer close(clientDone)
+		c, err := Dial(ln.Addr().String())
+		if err != nil {
+			t.Errorf("Client dial: %v", err)
+			return
+		}
+		_ = c.Close()
+		_, err = c.GetTLSConnectionState()
+		if err == nil {
+			t.Error("GetTLSConnectionState: expected error; got nil")
+			return
+		}
+	}()
+	<-clientDone
+	<-serverDone
+}
+
+func TestClient_GetTLSConnectionState_unableErr(t *testing.T) {
+	ln := newLocalListener(t)
+	defer func() {
+		_ = ln.Close()
+	}()
+	clientDone := make(chan bool)
+	serverDone := make(chan bool)
+	go func() {
+		defer close(serverDone)
+		c, err := ln.Accept()
+		if err != nil {
+			t.Errorf("Server accept: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Close()
+		}()
+		if err := serverHandle(c, t); err != nil {
+			t.Errorf("server error: %v", err)
+		}
+	}()
+	go func() {
+		defer close(clientDone)
+		c, err := Dial(ln.Addr().String())
+		if err != nil {
+			t.Errorf("Client dial: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Quit()
+		}()
+		c.tls = true
+		_, err = c.GetTLSConnectionState()
+		if err == nil {
+			t.Error("GetTLSConnectionState: expected error; got nil")
+			return
+		}
+	}()
+	<-clientDone
+	<-serverDone
+}
+
+func TestClient_HasConnection(t *testing.T) {
+	ln := newLocalListener(t)
+	defer func() {
+		_ = ln.Close()
+	}()
+	clientDone := make(chan bool)
+	serverDone := make(chan bool)
+	go func() {
+		defer close(serverDone)
+		c, err := ln.Accept()
+		if err != nil {
+			t.Errorf("Server accept: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Close()
+		}()
+		if err := serverHandle(c, t); err != nil {
+			t.Errorf("server error: %v", err)
+		}
+	}()
+	go func() {
+		defer close(clientDone)
+		c, err := Dial(ln.Addr().String())
+		if err != nil {
+			t.Errorf("Client dial: %v", err)
+			return
+		}
+		cfg := &tls.Config{ServerName: "example.com"}
+		testHookStartTLS(cfg) // set the RootCAs
+		if err := c.StartTLS(cfg); err != nil {
+			t.Errorf("StartTLS: %v", err)
+			return
+		}
+		if !c.HasConnection() {
+			t.Error("HasConnection: expected true; got false")
+			return
+		}
+		if err = c.Quit(); err != nil {
+			t.Errorf("closing connection failed: %s", err)
+			return
+		}
+		if c.HasConnection() {
+			t.Error("HasConnection: expected false; got true")
+		}
+	}()
+	<-clientDone
+	<-serverDone
+}
+
+func TestClient_SetDSNMailReturnOption(t *testing.T) {
+	ln := newLocalListener(t)
+	defer func() {
+		_ = ln.Close()
+	}()
+	clientDone := make(chan bool)
+	serverDone := make(chan bool)
+	go func() {
+		defer close(serverDone)
+		c, err := ln.Accept()
+		if err != nil {
+			t.Errorf("Server accept: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Close()
+		}()
+		if err := serverHandle(c, t); err != nil {
+			t.Errorf("server error: %v", err)
+		}
+	}()
+	go func() {
+		defer close(clientDone)
+		c, err := Dial(ln.Addr().String())
+		if err != nil {
+			t.Errorf("Client dial: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Quit()
+		}()
+		c.SetDSNMailReturnOption("foo")
+		if c.dsnmrtype != "foo" {
+			t.Errorf("SetDSNMailReturnOption: expected %s; got %s", "foo", c.dsnrntype)
+		}
+	}()
+	<-clientDone
+	<-serverDone
+}
+
+func TestClient_SetDSNRcptNotifyOption(t *testing.T) {
+	ln := newLocalListener(t)
+	defer func() {
+		_ = ln.Close()
+	}()
+	clientDone := make(chan bool)
+	serverDone := make(chan bool)
+	go func() {
+		defer close(serverDone)
+		c, err := ln.Accept()
+		if err != nil {
+			t.Errorf("Server accept: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Close()
+		}()
+		if err := serverHandle(c, t); err != nil {
+			t.Errorf("server error: %v", err)
+		}
+	}()
+	go func() {
+		defer close(clientDone)
+		c, err := Dial(ln.Addr().String())
+		if err != nil {
+			t.Errorf("Client dial: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Quit()
+		}()
+		c.SetDSNRcptNotifyOption("foo")
+		if c.dsnrntype != "foo" {
+			t.Errorf("SetDSNMailReturnOption: expected %s; got %s", "foo", c.dsnrntype)
+		}
+	}()
+	<-clientDone
+	<-serverDone
+}
+
+func TestClient_UpdateDeadline(t *testing.T) {
+	ln := newLocalListener(t)
+	defer func() {
+		_ = ln.Close()
+	}()
+	clientDone := make(chan bool)
+	serverDone := make(chan bool)
+	go func() {
+		defer close(serverDone)
+		c, err := ln.Accept()
+		if err != nil {
+			t.Errorf("Server accept: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Close()
+		}()
+		if err = serverHandle(c, t); err != nil {
+			t.Errorf("server error: %v", err)
+		}
+	}()
+	go func() {
+		defer close(clientDone)
+		c, err := Dial(ln.Addr().String())
+		if err != nil {
+			t.Errorf("Client dial: %v", err)
+			return
+		}
+		defer func() {
+			_ = c.Close()
+		}()
+		if !c.HasConnection() {
+			t.Error("HasConnection: expected true; got false")
+			return
+		}
+		if err = c.UpdateDeadline(time.Millisecond * 20); err != nil {
+			t.Errorf("failed to update deadline: %s", err)
+			return
+		}
+		time.Sleep(time.Millisecond * 50)
+		if !c.HasConnection() {
+			t.Error("HasConnection: expected true; got false")
+			return
+		}
+	}()
+	<-clientDone
+	<-serverDone
+}
+
 func newLocalListener(t *testing.T) net.Listener {
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
@@ -1685,6 +2036,8 @@ func serverHandle(c net.Conn, t *testing.T) error {
 			}
 			config := &tls.Config{Certificates: []tls.Certificate{keypair}}
 			return tf(config)
+		case "QUIT":
+			return nil
 		default:
 			t.Fatalf("unrecognized command: %q", s.Text())
 		}

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: