Update Msg documentation for clarity and RFC compliance

Enhanced the documentation for several Msg methods to provide clearer explanations and include relevant RFC references. This includes improved descriptions of functionality, parameter details, return values, and links to pertinent RFC sections.

msg.go

@@ -655,12 +655,26 @@ func (m *Msg) ReplyToFormat(name, addr string) error {
 	return m.ReplyTo(fmt.Sprintf(`"%s" <%s>`, name, addr))
 }
 
-// Subject sets the "Subject" header field of the Msg
+// Subject sets the "Subject" header for the Msg, specifying the topic of the message.
+//
+// This method takes a single string as input and sets it as the "Subject" of the email. The subject line provides
+// a brief summary of the content of the message, allowing recipients to quickly understand its purpose.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.5
 func (m *Msg) Subject(subj string) {
 	m.SetGenHeader(HeaderSubject, subj)
 }
 
-// SetMessageID generates a random message id for the mail
+// SetMessageID generates and sets a unique "Message-ID" header for the Msg.
+//
+// This method creates a "Message-ID" string using the current process ID, random numbers, and the hostname
+// of the machine. The generated ID helps uniquely identify the message in email systems, facilitating tracking
+// and preventing duplication. If the hostname cannot be retrieved, it defaults to "localhost.localdomain".
+//
+// The generated Message-ID follows the format
+// "<processID.randomNumberPrimary.randomNumberSecondary.randomString@hostname>".
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.4
 func (m *Msg) SetMessageID() {
 	hostname, err := os.Hostname()
 	if err != nil {
@@ -675,8 +689,14 @@ func (m *Msg) SetMessageID() {
 	m.SetMessageIDWithValue(messageID)
 }
 
-// GetMessageID returns the message ID of the Msg as string value. If no message ID
+// GetMessageID retrieves the "Message-ID" header from the Msg.
-// is set, an empty string will be returned
+//
+// This method checks if a "Message-ID" has been set in the message's generated headers. If a valid "Message-ID"
+// exists in the Msg, it returns the first occurrence of the header. If the "Message-ID" has not been set or
+// is empty, it returns an empty string. This allows other components to access the unique identifier for the
+// message, which is useful for tracking and referencing in email systems.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.4
 func (m *Msg) GetMessageID() string {
 	if msgidheader, ok := m.genHeader[HeaderMessageID]; ok {
 		if len(msgidheader) > 0 {
@@ -686,32 +706,67 @@ func (m *Msg) GetMessageID() string {
 	return ""
 }
 
-// SetMessageIDWithValue sets the message id for the mail
+// SetMessageIDWithValue sets the "Message-ID" header for the Msg using the provided messageID string.
+//
+// This method formats the input messageID by enclosing it in angle brackets ("<>") and sets it as the "Message-ID"
+// header in the message. The "Message-ID" is a unique identifier for the email, helping email clients and servers
+// to track and reference the message. There are no validations performed on the input messageID, so it should
+// be in a suitable format for use as a Message-ID.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.4
 func (m *Msg) SetMessageIDWithValue(messageID string) {
 	m.SetGenHeader(HeaderMessageID, fmt.Sprintf("<%s>", messageID))
 }
 
-// SetBulk sets the "Precedence: bulk" and "X-Auto-Response-Suppress: All" genHeaders which are
+// SetBulk sets the "Precedence: bulk" and "X-Auto-Response-Suppress: All" headers for the Msg,
-// recommended for automated mails like OOO replies
+// which are recommended for automated emails such as out-of-office replies.
-// See: https://www.rfc-editor.org/rfc/rfc2076#section-3.9
+//
-// See also: https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxcmail/ced68690-498a-4567-9d14-5c01f974d8b1#Appendix_A_Target_51
+// The "Precedence: bulk" header indicates that the message is a bulk email, and the "X-Auto-Response-Suppress: All"
+// header instructs mail servers and clients to suppress automatic responses to this message.
+// This is particularly useful for reducing unnecessary replies to automated notifications or replies.
+// For further details, refer to RFC 2076, Section 3.9, and Microsoft's documentation on
+// handling automated emails.
+//
+// https://www.rfc-editor.org/rfc/rfc2076#section-3.9
+//
+// https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxcmail/ced68690-498a-4567-9d14-5c01f974d8b1#Appendix_A_Target_51
 func (m *Msg) SetBulk() {
 	m.SetGenHeader(HeaderPrecedence, "bulk")
 	m.SetGenHeader(HeaderXAutoResponseSuppress, "All")
 }
 
-// SetDate sets the Date genHeader field to the current time in a valid format
+// SetDate sets the "Date" header for the Msg to the current time in a valid RFC 1123 format.
+//
+// This method retrieves the current time and formats it according to RFC 1123, ensuring that the "Date"
+// header is compliant with email standards. The "Date" header indicates when the message was created,
+// providing recipients with context for the timing of the email.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.3
 func (m *Msg) SetDate() {
 	now := time.Now().Format(time.RFC1123Z)
 	m.SetGenHeader(HeaderDate, now)
 }
 
-// SetDateWithValue sets the Date genHeader field to the provided time in a valid format
+// SetDateWithValue sets the "Date" header for the Msg using the provided time value in a valid RFC 1123 format.
+//
+// This method takes a `time.Time` value as input and formats it according to RFC 1123, ensuring that the "Date"
+// header is compliant with email standards. The "Date" header indicates when the message was created,
+// providing recipients with context for the timing of the email. This allows for setting a custom date
+// rather than using the current time.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.3
 func (m *Msg) SetDateWithValue(timeVal time.Time) {
 	m.SetGenHeader(HeaderDate, timeVal.Format(time.RFC1123Z))
 }
 
-// SetImportance sets the Msg Importance/Priority header to given Importance
+// SetImportance sets the "Importance" and "Priority" headers for the Msg to the specified Importance level.
+//
+// This method adjusts the email's importance based on the provided Importance value. If the importance level
+// is set to `ImportanceNormal`, no headers are modified. Otherwise, it sets the "Importance", "Priority",
+// "X-Priority", and "X-MSMail-Priority" headers accordingly, providing email clients with information on
+// how to prioritize the message. This allows the sender to indicate the significance of the email to recipients.
+//
+// https://datatracker.ietf.org/doc/html/rfc2156
 func (m *Msg) SetImportance(importance Importance) {
 	if importance == ImportanceNormal {
 		return
@@ -722,26 +777,48 @@ func (m *Msg) SetImportance(importance Importance) {
 	m.SetGenHeader(HeaderXMSMailPriority, importance.NumString())
 }
 
-// SetOrganization sets the provided string as Organization header for the Msg
+// SetOrganization sets the "Organization" header for the Msg to the specified organization string.
+//
+// This method allows you to specify the organization associated with the email sender. The "Organization"
+// header provides recipients with information about the organization that is sending the message.
+// This can help establish context and credibility for the email communication.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.4
 func (m *Msg) SetOrganization(org string) {
 	m.SetGenHeader(HeaderOrganization, org)
 }
 
-// SetUserAgent sets the User-Agent/X-Mailer header for the Msg
+// SetUserAgent sets the "User-Agent" and "X-Mailer" headers for the Msg to the specified user agent string.
+//
+// This method allows you to specify the user agent or mailer software used to send the email.
+// The "User-Agent" and "X-Mailer" headers provide recipients with information about the email client
+// or application that generated the message. This can be useful for identifying the source of the email,
+// particularly for troubleshooting or filtering purposes.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.7
 func (m *Msg) SetUserAgent(userAgent string) {
 	m.SetGenHeader(HeaderUserAgent, userAgent)
 	m.SetGenHeader(HeaderXMailer, userAgent)
 }
 
-// IsDelivered will return true if the Msg has been successfully delivered
+// IsDelivered indicates whether the Msg has been delivered.
+//
+// This method checks the internal state of the message to determine if it has been successfully
+// delivered. It returns true if the message is marked as delivered and false otherwise.
+// This can be useful for tracking the status of the email communication.
 func (m *Msg) IsDelivered() bool {
 	return m.isDelivered
 }
 
-// RequestMDNTo adds the Disposition-Notification-To header to request a MDN from the receiving end
+// RequestMDNTo adds the "Disposition-Notification-To" header to the Msg to request a Message Disposition
-// as described in RFC8098. It allows to provide a list recipient addresses.
+// Notification (MDN) from the receiving end, as specified in RFC 8098.
-// Address validation is performed
+//
-// See: https://www.rfc-editor.org/rfc/rfc8098.html
+// This method allows you to provide a list of recipient addresses to receive the MDN.
+// Each address is validated according to RFC 5322 standards. If ANY address is invalid, an error
+// will be returned indicating the parsing failure. If the "Disposition-Notification-To" header
+// is already set, it will be updated with the new list of addresses.
+//
+// https://datatracker.ietf.org/doc/html/rfc8098
 func (m *Msg) RequestMDNTo(rcpts ...string) error {
 	var addresses []string
 	for _, addrVal := range rcpts {
@@ -757,15 +834,26 @@ func (m *Msg) RequestMDNTo(rcpts ...string) error {
 	return nil
 }
 
-// RequestMDNToFormat adds the Disposition-Notification-To header to request a MDN from the receiving end
+// RequestMDNToFormat adds the "Disposition-Notification-To" header to the Msg to request a Message Disposition
-// as described in RFC8098. It allows to provide a recipient address with name and address and will format
+// Notification (MDN) from the receiving end, as specified in RFC 8098.
-// accordingly. Address validation is performed
+//
-// See: https://www.rfc-editor.org/rfc/rfc8098.html
+// This method allows you to provide a recipient address along with a name, formatting it appropriately.
+// Address validation is performed according to RFC 5322 standards. If the provided address is invalid,
+// an error will be returned. This method internally calls RequestMDNTo to handle the actual setting of the header.
+//
+// https://datatracker.ietf.org/doc/html/rfc8098
 func (m *Msg) RequestMDNToFormat(name, addr string) error {
 	return m.RequestMDNTo(fmt.Sprintf(`%s <%s>`, name, addr))
 }
 
-// RequestMDNAddTo adds an additional recipient to the recipient list of the MDN
+// RequestMDNAddTo adds an additional recipient to the "Disposition-Notification-To" header for the Msg.
+//
+// This method allows you to append a new recipient address to the existing list of recipients for the
+// MDN. The provided address is validated according to RFC 5322 standards. If the address is invalid,
+// an error will be returned indicating the parsing failure. If the "Disposition-Notification-To"
+// header is already set, the new recipient will be added to the existing list.
+//
+// https://datatracker.ietf.org/doc/html/rfc8098
 func (m *Msg) RequestMDNAddTo(rcpt string) error {
 	address, err := mail.ParseAddress(rcpt)
 	if err != nil {
@@ -780,14 +868,35 @@ func (m *Msg) RequestMDNAddTo(rcpt string) error {
 	return nil
 }
 
-// RequestMDNAddToFormat adds an additional formated recipient to the recipient list of the MDN
+// RequestMDNAddToFormat adds an additional formatted recipient to the "Disposition-Notification-To"
+// header for the Msg.
+//
+// This method allows you to specify a recipient address along with a name, formatting it appropriately
+// before adding it to the existing list of recipients for the MDN. The formatted address is validated
+// according to RFC 5322 standards. If the provided address is invalid, an error will be returned.
+// This method internally calls RequestMDNAddTo to handle the actual addition of the recipient.
+//
+// https://datatracker.ietf.org/doc/html/rfc8098
 func (m *Msg) RequestMDNAddToFormat(name, addr string) error {
 	return m.RequestMDNAddTo(fmt.Sprintf(`"%s" <%s>`, name, addr))
 }
 
-// GetSender returns the currently set envelope FROM address. If no envelope FROM is set it will use
+// GetSender returns the currently set envelope "FROM" address for the Msg. If no envelope
-// the first mail body FROM address. If useFullAddr is true, it will return the full address string
+// "FROM" address is set, it will use the first "FROM" address from the mail body. If the
-// including the address name, if set
+// useFullAddr parameter is true, it will return the full address string, including the name
+// if it is set.
+//
+// If neither the envelope "FROM" nor the body "FROM" addresses are available, it will return
+// an error indicating that no "FROM" address is present.
+//
+// Parameters:
+//   - useFullAddr: A boolean indicating whether to return the full address string (including
+//     the name) or just the email address.
+//
+// Returns:
+//   - The sender's address as a string and an error if applicable.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.2
 func (m *Msg) GetSender(useFullAddr bool) (string, error) {
 	from, ok := m.addrHeader[HeaderEnvelopeFrom]
 	if !ok || len(from) == 0 {
@@ -802,7 +911,18 @@ func (m *Msg) GetSender(useFullAddr bool) (string, error) {
 	return from[0].Address, nil
 }
 
-// GetRecipients returns a list of the currently set TO/CC/BCC addresses.
+// GetRecipients returns a list of the currently set "TO", "CC", and "BCC" addresses for the Msg.
+//
+// This method aggregates recipients from the "TO", "CC", and "BCC" headers and returns them as a
+// slice of strings. If no recipients are found in these headers, it will return an error indicating
+// that no recipient addresses are present.
+//
+// Returns:
+//   - A slice of strings containing the recipients' addresses and an error if applicable.
+//   - If there are no recipient addresses set, it will return an error indicating no recipient
+//     addresses are available.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.3
 func (m *Msg) GetRecipients() ([]string, error) {
 	var rcpts []string
 	for _, addressType := range []AddrHeader{HeaderTo, HeaderCc, HeaderBcc} {
@@ -820,12 +940,40 @@ func (m *Msg) GetRecipients() ([]string, error) {
 	return rcpts, nil
 }
 
-// GetAddrHeader returns the content of the requested address header of the Msg
+// GetAddrHeader returns the content of the requested address header for the Msg.
+//
+// This method retrieves the addresses associated with the specified address header. It returns a
+// slice of pointers to mail.Address structures representing the addresses found in the header.
+// If the requested header does not exist or contains no addresses, it will return nil.
+//
+// Parameters:
+//   - header: The AddrHeader enum value indicating which address header to retrieve (e.g., "TO",
+//     "CC", "BCC", etc.).
+//
+// Returns:
+//   - A slice of pointers to mail.Address structures containing the addresses from the specified
+//     header.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.6
 func (m *Msg) GetAddrHeader(header AddrHeader) []*mail.Address {
 	return m.addrHeader[header]
 }
 
-// GetAddrHeaderString returns the address string of the requested address header of the Msg
+// GetAddrHeaderString returns the address strings of the requested address header for the Msg.
+//
+// This method retrieves the addresses associated with the specified address header and returns them
+// as a slice of strings. Each address is formatted as a string, which includes both the name (if
+// available) and the email address. If the requested header does not exist or contains no addresses,
+// it will return an empty slice.
+//
+// Parameters:
+//   - header: The AddrHeader enum value indicating which address header to retrieve (e.g., "TO",
+//     "CC", "BCC", etc.).
+//
+// Returns:
+//   - A slice of strings containing the formatted addresses from the specified header.
+//
+// https://datatracker.ietf.org/doc/html/rfc5322#section-3.6
 func (m *Msg) GetAddrHeaderString(header AddrHeader) []string {
 	var addresses []string
 	for _, mh := range m.addrHeader[header] {