Add option for mailer to override mail headers

custom/conf/app.example.ini

@@ -1578,6 +1578,9 @@ LEVEL = Info
 ;; Prefix displayed before subject in mail
 ;SUBJECT_PREFIX =
 ;;
+;; Set the Return-Path header where bounced e-mails will be sent to.
+;RETURN_PATH =
+;;
 ;; Mail server protocol. One of "smtp", "smtps", "smtp+starttls", "smtp+unix", "sendmail", "dummy".
 ;; - sendmail: use the operating system's `sendmail` command instead of SMTP. This is common on Linux systems.
 ;; - dummy: send email messages to the log as a testing phase.

docs/content/administration/config-cheat-sheet.en-us.md

@@ -732,6 +732,7 @@ and
 - `ENABLE_HELO`: **true**: Enable HELO operation.
 - `HELO_HOSTNAME`: **(retrieved from system)**: HELO hostname.
 - `FROM`: **_empty_**: Mail from address, RFC 5322. This can be just an email address, or the "Name" \<email@example.com\> format.
+- `RETURN_PATH`: **_empty_**: Set the Return-Path header, to let mail clients reply to a different mail as it was send from. See [RFC 4021](https://www.rfc-editor.org/rfc/rfc4021#page-17) for more details.
 - `ENVELOPE_FROM`: **_empty_**: Address set as the From address on the SMTP mail envelope. Set to `<>` to send an empty address.
 - `SUBJECT_PREFIX`: **_empty_**: Prefix to be placed before e-mail subject lines.
 - `SENDMAIL_PATH`: **sendmail**: The location of sendmail on the operating system (can be command or full path).

modules/setting/mailer.go

@@ -26,6 +26,7 @@ type Mailer struct {
 	FromEmail            string `ini:"-"`
 	SendAsPlainText      bool   `ini:"SEND_AS_PLAIN_TEXT"`
 	SubjectPrefix        string `ini:"SUBJECT_PREFIX"`
+	ReturnPath           string `ini:"RETURN_PATH"`
 
 	// SMTP sender
 	Protocol             string `ini:"PROTOCOL"`

services/mailer/mailer.go

@@ -57,11 +57,14 @@ func (m *Message) ToMessage() *gomail.Message {
 		msg.SetHeader(header, m.Headers[header]...)
 	}
 
-	if len(setting.MailService.SubjectPrefix) > 0 {
+	if setting.MailService.SubjectPrefix != "" {
 		msg.SetHeader("Subject", setting.MailService.SubjectPrefix+" "+m.Subject)
 	} else {
 		msg.SetHeader("Subject", m.Subject)
 	}
+	if setting.MailService.ReturnPath != "" {
+		msg.SetHeader("Return-Path", setting.MailService.ReturnPath)
+	}
 	msg.SetDateHeader("Date", m.Date)
 	msg.SetHeader("X-Auto-Response-Suppress", "All")