Update EnvelopeFromFormat documentation in msg.go

Expanded the documentation for the EnvelopeFromFormat method to clarify its purpose, usage, and compliance with RFC 5322. Added details on how the envelope from address is used in SMTP communication and validation requirements.

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

@@ -20,6 +20,7 @@ const (
 	//
 	// 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"
 
@@ -33,12 +34,14 @@ const (
 	// 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.
-	// https://msopenspecs.azureedge.net/files/MS-XLOGIN/%5bMS-XLOGIN%5d.pdf
-	// https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00
 	//
 	// 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
@@ -47,11 +50,12 @@ const (
 	SMTPAuthNoAuth SMTPAuthType = ""
 
 	// SMTPAuthPlain is the "PLAIN" authentication mechanism as described in RFC 4616.
-	// https://datatracker.ietf.org/doc/html/rfc4616/
 	//
 	// 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.
@@ -59,16 +63,16 @@ const (
 	SMTPAuthXOAUTH2 SMTPAuthType = "XOAUTH2"
 
 	// SMTPAuthSCRAMSHA1 is the "SCRAM-SHA-1" SASL authentication mechanism as described in RFC 5802.
-	// https://datatracker.ietf.org/doc/html/rfc5802
 	//
 	// SCRAM-SHA-1 is still considered secure for certain applications, particularly when used as part
 	// of a challenge-response authentication mechanism (as we use it). However, it is generally
 	// recommended to prefer stronger alternatives like SCRAM-SHA-256(-PLUS), as SHA-1 has known
 	// vulnerabilities in other contexts, although it remains effective in HMAC constructions.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc5802
 	SMTPAuthSCRAMSHA1 SMTPAuthType = "SCRAM-SHA-1"
 
 	// SMTPAuthSCRAMSHA1PLUS is the "SCRAM-SHA-1-PLUS" SASL authentication mechanism as described in RFC 5802.
-	// https://datatracker.ietf.org/doc/html/rfc5802
 	//
 	// 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
@@ -78,18 +82,22 @@ const (
 	// of a challenge-response authentication mechanism (as we use it). However, it is generally
 	// recommended to prefer stronger alternatives like SCRAM-SHA-256(-PLUS), as SHA-1 has known
 	// vulnerabilities in other contexts, although it remains effective in HMAC constructions.
+	//
+	// https://datatracker.ietf.org/doc/html/rfc5802
 	SMTPAuthSCRAMSHA1PLUS SMTPAuthType = "SCRAM-SHA-1-PLUS"
 
 	// SMTPAuthSCRAMSHA256 is the "SCRAM-SHA-256" SASL authentication mechanism as described in RFC 7677.
+	//
 	// https://datatracker.ietf.org/doc/html/rfc7677
 	SMTPAuthSCRAMSHA256 SMTPAuthType = "SCRAM-SHA-256"
 
 	// SMTPAuthSCRAMSHA256PLUS is the "SCRAM-SHA-256-PLUS" SASL authentication mechanism as described in RFC 7677.
-	// https://datatracker.ietf.org/doc/html/rfc7677
 	//
 	// 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"
 )
 

client.go

@@ -44,26 +44,31 @@ const (
 
 	// DSNMailReturnHeadersOnly requests that only the message headers of the mail message are returned in
 	// a DSN (Delivery Status Notification).
+	//
 	// https://datatracker.ietf.org/doc/html/rfc1891#section-5.3
 	DSNMailReturnHeadersOnly DSNMailReturnOption = "HDRS"
 
 	// DSNMailReturnFull requests that the entire mail message is returned in any failed  DSN
 	// (Delivery Status Notification) issued for this recipient.
+	//
 	// https://datatracker.ietf.org/doc/html/rfc1891/#section-5.3
 	DSNMailReturnFull DSNMailReturnOption = "FULL"
 
 	// DSNRcptNotifyNever indicates that no DSN (Delivery Status Notifications) should be sent for the
 	// recipient under any condition.
+	//
 	// https://datatracker.ietf.org/doc/html/rfc1891/#section-5.1
 	DSNRcptNotifyNever DSNRcptNotifyOption = "NEVER"
 
 	// DSNRcptNotifySuccess indicates that the sender requests a DSN (Delivery Status Notification) if the
 	// message is successfully delivered.
+	//
 	// https://datatracker.ietf.org/doc/html/rfc1891/#section-5.1
 	DSNRcptNotifySuccess DSNRcptNotifyOption = "SUCCESS"
 
 	// DSNRcptNotifyFailure requests that a DSN (Delivery Status Notification) is issued if delivery of
 	// a message fails.
+	//
 	// https://datatracker.ietf.org/doc/html/rfc1891/#section-5.1
 	DSNRcptNotifyFailure DSNRcptNotifyOption = "FAILURE"
 
@@ -73,6 +78,7 @@ const (
 	// (as determined by the MTA at which the message is delayed), but the final delivery status (whether
 	// successful or failure) cannot be determined. The absence of the DELAY keyword in a NOTIFY parameter
 	// requests that a "delayed" DSN NOT be issued under any conditions.
+	//
 	// https://datatracker.ietf.org/doc/html/rfc1891/#section-5.1
 	DSNRcptNotifyDelay DSNRcptNotifyOption = "DELAY"
 )
@@ -87,11 +93,13 @@ type (
 
 	// DSNMailReturnOption is a type wrapper for a string and specifies the type of return content requested
 	// in a Delivery Status Notification (DSN).
+	//
 	// https://datatracker.ietf.org/doc/html/rfc1891/
 	DSNMailReturnOption string
 
 	// DSNRcptNotifyOption is a type wrapper for a string and specifies the notification options for a
 	// recipient in DSNs.
+	//
 	// https://datatracker.ietf.org/doc/html/rfc1891/
 	DSNRcptNotifyOption string
 
@@ -167,6 +175,7 @@ type (
 		smtpClient *smtp.Client
 
 		// tlspolicy defines the TLSPolicy configuration the Client uses for the STARTTLS protocol.
+		//
 		// https://datatracker.ietf.org/doc/html/rfc3207#section-2
 		tlspolicy TLSPolicy
 
@@ -180,6 +189,7 @@ type (
 		user string
 
 		// useSSL indicates whether to use SSL/TLS encryption for network communication.
+		//
 		// https://datatracker.ietf.org/doc/html/rfc8314
 		useSSL bool
 	}
@@ -424,10 +434,11 @@ func WithPassword(password string) Option {
 
 // WithDSN enables DSN (Delivery Status Notifications) for the Client as described in the RFC 1891. DSN
 // only work if the server supports them.
-// https://datatracker.ietf.org/doc/html/rfc1891
 //
 // By default we set DSNMailReturnOption to DSNMailReturnFull and DSNRcptNotifyOption to DSNRcptNotifySuccess
 // and DSNRcptNotifyFailure.
+//
+// https://datatracker.ietf.org/doc/html/rfc1891
 func WithDSN() Option {
 	return func(c *Client) error {
 		c.requestDSN = true
@@ -439,9 +450,10 @@ func WithDSN() Option {
 
 // WithDSNMailReturnType enables DSN (Delivery Status Notifications) for the Client as described in the
 // RFC 1891. DSN only work if the server supports them.
-// https://datatracker.ietf.org/doc/html/rfc1891
 //
 // It will set the DSNMailReturnOption to the provided value.
+//
+// https://datatracker.ietf.org/doc/html/rfc1891
 func WithDSNMailReturnType(option DSNMailReturnOption) Option {
 	return func(c *Client) error {
 		switch option {
@@ -459,9 +471,10 @@ func WithDSNMailReturnType(option DSNMailReturnOption) Option {
 
 // WithDSNRcptNotifyType enables DSN (Delivery Status Notifications) for the Client as described in the
 // RFC 1891. DSN only work if the server supports them.
-// https://datatracker.ietf.org/doc/html/rfc1891
 //
 // It will set the DSNRcptNotifyOption to the provided values.
+//
+// https://datatracker.ietf.org/doc/html/rfc1891
 func WithDSNRcptNotifyType(opts ...DSNRcptNotifyOption) Option {
 	return func(c *Client) error {
 		var rcptOpts []string

client_119.go

@@ -9,7 +9,10 @@ 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 to the Msg in case there of a transmission or delivery
+// error.
 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 +11,10 @@ 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 to the Msg in case there of a transmission or delivery
+// error.
 func (c *Client) Send(messages ...*Msg) (returnErr error) {
 	if err := c.checkConn(); err != nil {
 		returnErr = &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}

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 +18,13 @@ import (
 	"strings"
 )
 
-// EMLToMsgFromString will parse a given EML string and returns a pre-filled Msg pointer
+// EMLToMsgFromString will parse a given EML string and returns a pre-filled Msg pointer.
 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
-// Msg pointer
+// EMLToMsgFromReader will parse a reader that holds EML content and returns a pre-filled Msg pointer.
 func EMLToMsgFromReader(reader io.Reader) (*Msg, error) {
 	msg := &Msg{
 		addrHeader:    make(map[AddrHeader][]*netmail.Address),
@@ -46,8 +45,7 @@ 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
-// pre-filled Msg pointer
+// EMLToMsgFromFile will open and parse a .eml file at a provided file path and returns a pre-filled Msg pointer.
 func EMLToMsgFromFile(filePath string) (*Msg, error) {
 	msg := &Msg{
 		addrHeader:    make(map[AddrHeader][]*netmail.Address),
@@ -68,7 +66,7 @@ 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.
 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 +77,7 @@ 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.
 func readEML(filePath string) (*netmail.Message, *bytes.Buffer, error) {
 	fileHandle, err := os.Open(filePath)
 	if err != nil {
@@ -91,7 +89,7 @@ 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.
 func readEMLFromReader(reader io.Reader) (*netmail.Message, *bytes.Buffer, error) {
 	parsedMsg, err := netmail.ReadMessage(reader)
 	if err != nil {
@@ -106,8 +104,8 @@ 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
-// according settings in the Msg
+// parseEMLHeaders will check the EML headers for the most common headers and set the according settings
+// in the Msg.
 func parseEMLHeaders(mailHeader *netmail.Header, msg *Msg) error {
 	commonHeaders := []Header{
 		HeaderContentType, HeaderImportance, HeaderInReplyTo, HeaderListUnsubscribe,
@@ -175,7 +173,7 @@ 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 a EML based on the different content types and encodings.
 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 +210,11 @@ 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 mails.
 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())
@@ -349,7 +348,7 @@ ReadNextPart:
 	return nil
 }
 
-// parseEMLEncoding parses and determines the encoding of the message
+// parseEMLEncoding parses and determines the encoding of the message.
 func parseEMLEncoding(mailHeader *netmail.Header, msg *Msg) {
 	if value := mailHeader.Get(HeaderContentTransferEnc.String()); value != "" {
 		switch {
@@ -363,7 +362,7 @@ 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.
 func parseEMLContentTypeCharset(mailHeader *netmail.Header, msg *Msg) {
 	if value := mailHeader.Get(HeaderContentType.String()); value != "" {
 		contentType, optional := parseMultiPartHeader(value)
@@ -377,7 +376,7 @@ 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.
 func handleEMLMultiPartBase64Encoding(multiPartData []byte, part *Part) error {
 	part.SetEncoding(EncodingB64)
 	content, err := base64.StdEncoding.DecodeString(string(multiPartData))
@@ -388,8 +387,7 @@ func handleEMLMultiPartBase64Encoding(multiPartData []byte, part *Part) error {
 	return nil
 }
 
-// parseMultiPartHeader parses a multipart header and returns the value and optional parts as
-// separate map
+// parseMultiPartHeader parses a multipart header and returns the value and optional parts as separate map.
 func parseMultiPartHeader(multiPartHeader string) (header string, optional map[string]string) {
 	optional = make(map[string]string)
 	headerSplit := strings.SplitN(multiPartHeader, ";", 2)
@@ -404,7 +402,7 @@ 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.
 func parseEMLAttachmentEmbed(contentDisposition []string, multiPart *multipart.Part, msg *Msg) error {
 	cdType, optional := parseMultiPartHeader(contentDisposition[0])
 	filename := "generic.attachment"

encoding.go

@@ -4,171 +4,198 @@
 
 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
-const (
-	// MIME10 is the MIME Version 1.0
-	MIME10 MIMEVersion = "1.0"
-)
+// MIME10 represents the MIME version "1.0" used in email messages.
+const 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"
-	TypePGPSignature         ContentType = "application/pgp-signature"
-	TypePGPEncrypted         ContentType = "application/pgp-encrypted"
-	TypeTextHTML             ContentType = "text/html"
-	TypeTextPlain            ContentType = "text/plain"
+
+	// TypeMultipartMixed represents the MIME type for a multipart message containing different parts.
+	TypeMultipartMixed ContentType = "multipart/mixed"
+
+	// TypeMultipartRelated represents the MIME type for a multipart message where each part is a related file
+	// 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.
 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.
 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.
 func (e Encoding) String() string {
 	return string(e)
 }

file.go

@@ -9,10 +9,11 @@ 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 like content type, description, encoding, headers, name, and
+// writer function. This can either be an attachment or an embedded file for a Msg.
 type File struct {
 	ContentType ContentType
 	Desc        string
@@ -22,32 +23,35 @@ 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.
 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.
 func WithFileName(name string) FileOption {
 	return func(f *File) {
 		f.Name = name
 	}
 }
 
-// WithFileDescription sets an optional file description of the File that will be
-// added as Content-Description part
+// WithFileDescription sets an optional file description for the File. The description is used in the
+// Content-Description header of the MIME output.
 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
-// 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
-// is provided as argument, the function will automatically override back to Base64
+// WithFileEncoding sets the encoding type for a file.
+//
+// By default one should always use Base64 encoding for attachments and embeds, but there might be exceptions in
+// which this might come handy.
+//
+// Note: that quoted-printable must never be used for attachments or embeds. If EncodingQP is provided as encoding
+// to this method, it will be automatically overwritten with EncodingB64.
 func WithFileEncoding(encoding Encoding) FileOption {
 	return func(f *File) {
 		if encoding == EncodingQP {
@@ -58,23 +62,23 @@ 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
-// could not be guessed. In some cases, however, it might be needed to force
-// this to a specific type. For such situations this override method can
-// be used
+//
+// By default we will try to guess the file type and its corresponding content type and fall back to
+// application/octet-stream if the file type, if no matching type could be guessed. This FileOption can
+// be used to override this type, in case a specific type is required.
 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 given MIME header field for the File.
 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. It returns the header value and a boolean
+// indicating whether the header was present or not.
 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/text v0.18.0
+	golang.org/x/crypto v0.28.0
+	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 v0.27.0/go.mod h1:1Xngt8kV6Dvbssa53Ziq6Eqn0HqbZi5Z6R0ZpwQzt70=
+golang.org/x/crypto v0.28.0 h1:GBDwsMXVQi34v5CCYUm2jkJvu4cbtru2U4TN2PSyQnw=
+golang.org/x/crypto v0.28.0/go.mod h1:rmgy+3RHxRZMyY0jjAJShp2zgEdOqj2AO7U0pYmeQ7U=
 golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
 golang.org/x/mod v0.8.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
 golang.org/x/mod v0.12.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
@@ -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 v0.18.0/go.mod h1:BuEKDfySbSR4drPmRPG/7iBdf8hvFMuRexcpahXilzY=
+golang.org/x/text v0.19.0 h1:kTxAhCbGbxhK0IwgSKiMO5awPoDQ0RpfiVYBfK860YM=
+golang.org/x/text v0.19.0/go.mod h1:BuEKDfySbSR4drPmRPG/7iBdf8hvFMuRexcpahXilzY=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
 golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=

header.go

@@ -4,129 +4,140 @@
 
 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
-	// See: https://www.rfc-editor.org/rfc/rfc822#section-5.1
+	// HeaderDate represents the "Date" field.
+	// https://datatracker.ietf.org/doc/html/rfc822#section-5.1
 	HeaderDate Header = "Date"
 
-	// HeaderDispositionNotificationTo is the MDN header as described in RFC8098
-	// See: https://www.rfc-editor.org/rfc/rfc8098.html#section-2.1
+	// HeaderDispositionNotificationTo is the MDN header as described in RFC 8098.
+	// 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
-	// See: https://www.rfc-editor.org/rfc/rfc1036#section-2.1.5
+	// HeaderMessageID represents the "Message-ID" field for message identification.
+	// 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
-	// See: https://datatracker.ietf.org/doc/html/rfc2045#section-4
+	// HeaderMIMEVersion represents the "MIME-Version" field as per RFC 2045.
+	// 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
-	// It's not included in the mail body but only used by the Client for the envelope
+	// HeaderEnvelopeFrom is the envelope FROM header field.
+	//
+	// 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, mapping ImportanceHigh and
+// ImportanceUrgent to "1" and others to "0".
 func (i Importance) NumString() string {
 	switch i {
 	case ImportanceNonUrgent:
@@ -142,7 +153,8 @@ 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, mapping ImportanceHigh and
+// ImportanceUrgent to "1" and others to "5".
 func (i Importance) XPrioString() string {
 	switch i {
 	case ImportanceNonUrgent:
@@ -158,7 +170,8 @@ 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.
 func (i Importance) String() string {
 	switch i {
 	case ImportanceNonUrgent:
@@ -174,12 +187,13 @@ 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.
 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.
 func (a AddrHeader) String() string {
 	return string(a)
 }

msg.go

@@ -24,111 +24,134 @@ import (
 )
 
 var (
-	// ErrNoFromAddress should be used when a FROM address is requrested but not set
+	// ErrNoFromAddress indicates that the FROM address is not set, which is required.
 	ErrNoFromAddress = errors.New("no FROM address set")
 
-	// ErrNoRcptAddresses should be used when the list of RCPTs is empty
+	// ErrNoRcptAddresses indicates that no recipient addresses have been set.
 	ErrNoRcptAddresses = errors.New("no recipient addresses set")
 )
 
 const (
-	// errTplExecuteFailed is issued when the template execution was not successful
+	// errTplExecuteFailed indicates that the execution of a template has failed, including the underlying error.
 	errTplExecuteFailed = "failed to execute template: %w"
 
-	// errTplPointerNil is issued when a template pointer is expected but it is nil
+	// errTplPointerNil indicates that a template pointer is nil, which prevents further template execution or
+	// processing.
 	errTplPointerNil = "template pointer is nil"
 
-	// errParseMailAddr is used when a mail address could not be validated
+	// errParseMailAddr indicates that parsing of a mail address has failed, including the problematic address
+	// and error.
 	errParseMailAddr = "failed to parse mail address %q: %w"
 )
 
 const (
-	// NoPGP indicates that a message should not be treated as PGP encrypted
-	// or signed and is the default value for a message
+	// NoPGP indicates that a message should not be treated as PGP encrypted or signed and is the default value
+	// for a message
 	NoPGP PGPType = iota
-	// PGPEncrypt indicates that a message should be treated as PGP encrypted
-	// This works closely together with the corresponding go-mail-middleware
+	// PGPEncrypt indicates that a message should be treated as PGP encrypted. This works closely together with
+	// the corresponding go-mail-middleware.
 	PGPEncrypt
-	// PGPSignature indicates that a message should be treated as PGP signed
-	// This works closely together with the corresponding go-mail-middleware
+	// PGPSignature indicates that a message should be treated as PGP signed. This works closely together with
+	// the corresponding go-mail-middleware.
 	PGPSignature
 )
 
-// MiddlewareType is the type description of the Middleware and needs to be returned
-// in the Middleware interface by the Type method
+// MiddlewareType is a type wrapper for a string. It describes the type of the Middleware and needs to be
+// returned by the Middleware.Type method to satisfy the Middleware interface.
 type MiddlewareType string
 
-// Middleware is an interface to define a function to apply to Msg before sending
+// Middleware represents the interface for modifying or handling email messages. A Middleware allows the user to
+// alter a Msg before it is finally processed. Multiple Middleware can be applied to a Msg.
+//
+// Type returns a unique MiddlewareType. It describes the type of Middleware and makes sure that
+// a Middleware is only applied once.
+// Handle performs all the processing to the Msg. It always needs to return a Msg back.
 type Middleware interface {
 	Handle(*Msg) *Msg
 	Type() MiddlewareType
 }
 
-// PGPType is a type alias for a int representing a type of PGP encryption
-// or signature
+// PGPType is a type wrapper for an int, representing a type of PGP encryption or signature.
 type PGPType int
 
-// Msg is the mail message struct
+// Msg represents an email message with various headers, attachments, and encoding settings.
+//
+// The Msg is the central part of go-mail. It provided a lot of methods that you would expect in a mail
+// user agent (MUA). Msg satisfies the io.WriterTo and io.Reader interfaces.
 type Msg struct {
-	// addrHeader is a slice of strings that the different mail AddrHeader fields
+	// addrHeader holds a mapping between AddrHeader keys and their corresponding slices of mail.Address pointers.
 	addrHeader map[AddrHeader][]*mail.Address
 
-	// attachments represent the different attachment File of the Msg
+	// attachments holds a list of File pointers that represent files either as attachments or embeds files in
+	// a Msg.
 	attachments []*File
 
-	// boundary is the MIME content boundary
+	// boundary represents the delimiter for separating parts in a multipart message.
 	boundary string
 
-	// charset represents the charset of the mail (defaults to UTF-8)
+	// charset represents the Charset of the Msg.
+	//
+	// By default we set CharsetUTF8 for a Msg unless overridden by a corresponding MsgOption.
 	charset Charset
 
-	// embeds represent the different embedded File of the Msg
+	// embeds contains a slice of File pointers representing the embedded files in a Msg.
 	embeds []*File
 
-	// encoder represents a mime.WordEncoder from the std lib
+	// encoder is a mime.WordEncoder used to encode strings (such as email headers) using a specified
+	// Encoding.
 	encoder mime.WordEncoder
 
-	// encoding represents the message encoding (the encoder will be a corresponding WordEncoder)
+	// encoding specifies the type of Encoding used for email messages and/or parts.
 	encoding Encoding
 
-	// genHeader is a slice of strings that the different generic mail Header fields
+	// genHeader is a map where the keys are email headers (of type Header) and the values are slices of strings
+	// representing header values.
 	genHeader map[Header][]string
 
-	// isDelivered signals if a message has been delivered or not
+	// isDelivered indicates wether the Msg has been delivered.
 	isDelivered bool
 
-	// middlewares is the list of middlewares to apply to the Msg before sending in FIFO order
+	// middlewares is a slice of Middleware used for modifying or handling messages before they are processed.
+	//
+	// middlewares are processed in FIFO order.
 	middlewares []Middleware
 
-	// mimever represents the MIME version
+	// mimever represents the MIME version used in a Msg.
 	mimever MIMEVersion
 
-	// parts represent the different parts of the Msg
+	// parts is a slice that holds pointers to Part structures, which represent different parts of a Msg.
 	parts []*Part
 
-	// preformHeader is a slice of strings that the different generic mail Header fields
-	// of which content is already preformated and will not be affected by the automatic line
-	// breaks
+	// preformHeader maps Header types to their already preformatted string values.
+	//
+	// Preformatted Header values will not be affected by automatic line breaks.
 	preformHeader map[Header]string
 
 	// pgptype indicates that a message has a PGPType assigned and therefore will generate
-	// different Content-Type settings in the msgWriter
+	// different Content-Type settings in the msgWriter.
 	pgptype PGPType
 
-	// sendError holds the SendError in case a Msg could not be delivered during the Client.Send operation
+	// sendError represents an error encountered during the process of sending a Msg during the
+	// Client.Send operation.
+	//
+	// sendError will hold an error of type SendError.
 	sendError error
 
-	// noDefaultUserAgent indicates whether the default User Agent will be excluded for the Msg when it's sent.
+	// noDefaultUserAgent indicates whether the default User-Agent will be omitted for the Msg when it is
+	// being sent.
+	//
+	// This can be useful in scenarios where headers are conditionally passed based on receipt - i. e. SMTP proxies.
 	noDefaultUserAgent bool
 }
 
-// SendmailPath is the default system path to the sendmail binary
+// SendmailPath is the default system path to the sendmail binary - at least on standard Unix-like OS.
 const SendmailPath = "/usr/sbin/sendmail"
 
-// MsgOption returns a function that can be used for grouping Msg options
+// MsgOption is a function type that modifies a Msg instance during its creation or initialization.
 type MsgOption func(*Msg)
 
-// NewMsg returns a new Msg pointer
+// NewMsg creates a new email message with optional MsgOption functions that customize various aspects of the
+// message.
 func NewMsg(opts ...MsgOption) *Msg {
 	msg := &Msg{
 		addrHeader:    make(map[AddrHeader][]*mail.Address),
@@ -139,7 +162,7 @@ func NewMsg(opts ...MsgOption) *Msg {
 		mimever:       MIME10,
 	}
 
-	// Override defaults with optionally provided MsgOption functions
+	// Override defaults with optionally provided MsgOption functions.
 	for _, option := range opts {
 		if option == nil {
 			continue
@@ -153,101 +176,133 @@ func NewMsg(opts ...MsgOption) *Msg {
 	return msg
 }
 
-// WithCharset overrides the default message charset
+// WithCharset sets the Charset type for a Msg during its creation or initialization.
 func WithCharset(c Charset) MsgOption {
 	return func(m *Msg) {
 		m.charset = c
 	}
 }
 
-// WithEncoding overrides the default message encoding
+// WithEncoding sets the Encoding type for a Msg during its creation or initialization.
 func WithEncoding(e Encoding) MsgOption {
 	return func(m *Msg) {
 		m.encoding = e
 	}
 }
 
-// WithMIMEVersion overrides the default MIME version
+// WithMIMEVersion sets the MIMEVersion type for a Msg during its creation or initialization.
+//
+// Note that in the context of email, MIME Version 1.0 is the only officially standardized and supported
+// version. While MIME has been updated and extended over time (via various RFCs), these updates and extensions
+// do not introduce new MIME versions; they refine or add features within the framework of MIME 1.0.
+// Therefore there should be no reason to ever use this MsgOption.
+// https://datatracker.ietf.org/doc/html/rfc1521
+//
+// https://datatracker.ietf.org/doc/html/rfc2045
+//
+// https://datatracker.ietf.org/doc/html/rfc2049
 func WithMIMEVersion(mv MIMEVersion) MsgOption {
 	return func(m *Msg) {
 		m.mimever = mv
 	}
 }
 
-// WithBoundary overrides the default MIME boundary
+// WithBoundary sets the boundary of a Msg to the provided string value during its creation or initialization.
+//
+// Note that by default we create random MIME boundaries. This should only be used if a specific boundary is
+// required.
 func WithBoundary(b string) MsgOption {
 	return func(m *Msg) {
 		m.boundary = b
 	}
 }
 
-// WithMiddleware add the given middleware in the end of the list of the client middlewares
+// WithMiddleware adds the given Middleware to the end of the list of the Client middlewares slice. Middleware
+// are processed in FIFO order.
 func WithMiddleware(mw Middleware) MsgOption {
 	return func(m *Msg) {
 		m.middlewares = append(m.middlewares, mw)
 	}
 }
 
-// WithPGPType overrides the default PGPType of the message
+// WithPGPType sets the PGP type for the Msg during its creation or initialization, determining the encryption or
+// signature method.
 func WithPGPType(pt PGPType) MsgOption {
 	return func(m *Msg) {
 		m.pgptype = pt
 	}
 }
 
-// WithNoDefaultUserAgent configures the Msg to not use the default User Agent
+// WithNoDefaultUserAgent disables the inclusion of a default User-Agent header in the Msg during its creation or
+// initialization.
 func WithNoDefaultUserAgent() MsgOption {
 	return func(m *Msg) {
 		m.noDefaultUserAgent = true
 	}
 }
 
-// SetCharset sets the encoding charset of the Msg
+// SetCharset sets or overrides the currently set encoding charset of the Msg.
 func (m *Msg) SetCharset(c Charset) {
 	m.charset = c
 }
 
-// SetEncoding sets the encoding of the Msg
+// SetEncoding sets or overrides the currently set Encoding of the Msg.
 func (m *Msg) SetEncoding(e Encoding) {
 	m.encoding = e
 	m.setEncoder()
 }
 
-// SetBoundary sets the boundary of the Msg
+// SetBoundary sets or overrides the currently set boundary of the Msg.
+//
+// Note that by default we create random MIME boundaries. This should only be used if a specific boundary is
+// required.
 func (m *Msg) SetBoundary(b string) {
 	m.boundary = b
 }
 
-// SetMIMEVersion sets the MIME version of the Msg
+// SetMIMEVersion sets or overrides the currently set MIME version of the Msg.
+//
+// Note that in the context of email, MIME Version 1.0 is the only officially standardized and supported
+// version. While MIME has been updated and extended over time (via various RFCs), these updates and extensions
+// do not introduce new MIME versions; they refine or add features within the framework of MIME 1.0.
+// Therefore there should be no reason to ever use this MsgOption.
+//
+// https://datatracker.ietf.org/doc/html/rfc1521
+//
+// https://datatracker.ietf.org/doc/html/rfc2045
+//
+// https://datatracker.ietf.org/doc/html/rfc2049
 func (m *Msg) SetMIMEVersion(mv MIMEVersion) {
 	m.mimever = mv
 }
 
-// SetPGPType sets the PGPType of the Msg
+// SetPGPType sets or overrides the currently set PGP type for the Msg, determining the encryption or
+// signature method.
 func (m *Msg) SetPGPType(t PGPType) {
 	m.pgptype = t
 }
 
-// Encoding returns the currently set encoding of the Msg
+// Encoding returns the currently set Encoding of the Msg as string.
 func (m *Msg) Encoding() string {
 	return m.encoding.String()
 }
 
-// Charset returns the currently set charset of the Msg
+// Charset returns the currently set Charset of the Msg as string.
 func (m *Msg) Charset() string {
 	return m.charset.String()
 }
 
-// SetHeader sets a generic header field of the Msg
-// For adding address headers like "To:" or "From", see SetAddrHeader
+// SetHeader sets a generic header field of the Msg.
 //
-// Deprecated: This method only exists for compatibility reason. Please use SetGenHeader instead
+// Deprecated: This method only exists for compatibility reason. Please use SetGenHeader instead.
+// For adding address headers like "To:" or "From", use SetAddrHeader instead.
 func (m *Msg) SetHeader(header Header, values ...string) {
 	m.SetGenHeader(header, values...)
 }
 
-// SetGenHeader sets a generic header field of the Msg
-// For adding address headers like "To:" or "From", see SetAddrHeader
+// SetGenHeader sets a generic header field of the Msg to the provided list of values.
+//
+// Note: for adding email address related headers (like "To:" or "From") use SetAddrHeader instead.
 func (m *Msg) SetGenHeader(header Header, values ...string) {
 	if m.genHeader == nil {
 		m.genHeader = make(map[Header][]string)
@@ -258,26 +313,24 @@ func (m *Msg) SetGenHeader(header Header, values ...string) {
 	m.genHeader[header] = values
 }
 
-// SetHeaderPreformatted sets a generic header field of the Msg which content is
-// already preformated.
+// SetHeaderPreformatted sets a generic header field of the Msg, which content is already preformatted.
 //
-// Deprecated: This method only exists for compatibility reason. Please use
-// SetGenHeaderPreformatted instead
+// Deprecated: This method only exists for compatibility reason. Please use SetGenHeaderPreformatted instead.
 func (m *Msg) SetHeaderPreformatted(header Header, value string) {
 	m.SetGenHeaderPreformatted(header, value)
 }
 
-// SetGenHeaderPreformatted sets a generic header field of the Msg which content is
-// already preformated.
+// SetGenHeaderPreformatted sets a generic header field of the Msg which content is already preformated.
 //
-// This method does not take a slice of values but only a single value. This is
-// due to the fact, that we do not perform any content alteration and expect the
-// user has already done so
+// This method does not take a slice of values but only a single value. The reason for this is that we do not
+// perform any content alteration on these kind of headers and expect the user to have already taken care of
+// any kind of formatting required for the header.
 //
-// **Please note:** This method should be used only as a last resort. Since the
-// user is respondible for the formating of the message header, go-mail cannot
-// guarantee the fully compliance with the RFC 2822. It is recommended to use
-// SetGenHeader instead.
+// Note: This method should be used only as a last resort. Since the user is respondible for the formatting of
+// the message header, we cannot guarantee any compliance with the RFC 2822. It is advised to use SetGenHeader
+// instead.
+//
+// https://datatracker.ietf.org/doc/html/rfc2822
 func (m *Msg) SetGenHeaderPreformatted(header Header, value string) {
 	if m.preformHeader == nil {
 		m.preformHeader = make(map[Header]string)
@@ -285,7 +338,13 @@ func (m *Msg) SetGenHeaderPreformatted(header Header, value string) {
 	m.preformHeader[header] = value
 }
 
-// SetAddrHeader sets an address related header field of the Msg
+// SetAddrHeader sets the specified AddrHeader for the Msg to the given values.
+//
+// Addresses are parsed according to RFC 5322. If parsing of ANY of the provided values fails,
+// and error is returned. If you cannot guarantee that all provided values are valid, you can
+// use SetAddrHeaderIgnoreInvalid instead, which will skip any parsing error silently.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.4
 func (m *Msg) SetAddrHeader(header AddrHeader, values ...string) error {
 	if m.addrHeader == nil {
 		m.addrHeader = make(map[AddrHeader][]*mail.Address)
@@ -309,8 +368,12 @@ func (m *Msg) SetAddrHeader(header AddrHeader, values ...string) error {
 	return nil
 }
 
-// SetAddrHeaderIgnoreInvalid sets an address related header field of the Msg and ignores invalid address
-// in the validation process
+// SetAddrHeaderIgnoreInvalid sets the specified AddrHeader for the Msg to the given values.
+//
+// Addresses are parsed according to RFC 5322. If parsing of ANY of the provided values fails,
+// the error is ignored and the address omiitted from the address list.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.4
 func (m *Msg) SetAddrHeaderIgnoreInvalid(header AddrHeader, values ...string) {
 	var addresses []*mail.Address
 	for _, addrVal := range values {
@@ -323,14 +386,26 @@ func (m *Msg) SetAddrHeaderIgnoreInvalid(header AddrHeader, values ...string) {
 	m.addrHeader[header] = addresses
 }
 
-// EnvelopeFrom takes and validates a given mail address and sets it as envelope "FROM"
-// addrHeader of the Msg
+// EnvelopeFrom sets the envelope from address for the Msg.
+//
+// The HeaderEnvelopeFrom address 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. The provided address is validated
+// according to RFC 5322 and will return an error if the validation fails.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.4
 func (m *Msg) EnvelopeFrom(from string) error {
 	return m.SetAddrHeader(HeaderEnvelopeFrom, from)
 }
 
-// EnvelopeFromFormat takes a name and address, formats them RFC5322 compliant and stores them as
-// the envelope FROM address header field
+// EnvelopeFromFormat sets the provided name and mail address as HeaderEnvelopeFrom for the Msg.
+//
+// The HeaderEnvelopeFrom address 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. The provided name and address adre
+// validated according to RFC 5322 and will return an error if the validation fails.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.4
 func (m *Msg) EnvelopeFromFormat(name, addr string) error {
 	return m.SetAddrHeader(HeaderEnvelopeFrom, fmt.Sprintf(`"%s" <%s>`, name, addr))
 }

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
 }
@@ -516,8 +529,7 @@ func (c *Client) Quit() error {
 	}
 	c.mutex.Lock()
 	err = c.Text.Close()
-	c.Text = nil
-	c.conn = nil
+	c.isConnected = false
 	c.mutex.Unlock()
 
 	return err
@@ -558,17 +570,18 @@ func (c *Client) SetDSNRcptNotifyOption(d string) {
 // Returns true if the `conn` field is not nil, indicating an active connection.
 func (c *Client) HasConnection() bool {
 	c.mutex.RLock()
-	conn := c.conn
+	isConn := c.isConnected
 	c.mutex.RUnlock()
-	return conn != nil
+	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
 }
 
@@ -578,17 +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())
 		}