Add option for mailer to override mail headers

custom/conf/app.example.ini

@@ -1687,6 +1687,16 @@ LEVEL = Info
 ;; convert \r\n to \n for Sendmail
 ;SENDMAIL_CONVERT_CRLF = true
 
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;[mailer.override_header]
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; This is empty by default, use it only if you know what you need it for.
+;Reply-To = test@example.com, test2@example.com
+;Content-Type = text/html; charset=utf-8
+;In-Reply-To =
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;[email.incoming]

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

@@ -724,11 +724,13 @@ Define allowed algorithms and their minimum key length (use -1 to disable a type
 
 ## Mailer (`mailer`)
 
-⚠️ This section is for Gitea 1.18 and later. If you are using Gitea 1.17 or older,
+:::warning
+This section is for Gitea 1.18 and later. If you are using Gitea 1.17 or older,
 please refer to
 [Gitea 1.17 app.ini example](https://github.com/go-gitea/gitea/blob/release/v1.17/custom/conf/app.example.ini)
 and
 [Gitea 1.17 configuration document](https://github.com/go-gitea/gitea/blob/release/v1.17/docs/content/doc/advanced/config-cheat-sheet.en-us.md)
+:::
 
 - `ENABLED`: **false**: Enable to use a mail service.
 - `PROTOCOL`: **_empty_**: Mail server protocol. One of "smtp", "smtps", "smtp+starttls", "smtp+unix", "sendmail", "dummy". _Before 1.18, this was inferred from a combination of `MAILER_TYPE` and `IS_TLS_ENABLED`._
@@ -761,6 +763,21 @@ and
 - `SEND_BUFFER_LEN`: **100**: Buffer length of mailing queue. **DEPRECATED** use `LENGTH` in `[queue.mailer]`
 - `SEND_AS_PLAIN_TEXT`: **false**: Send mails only in plain text, without HTML alternative.
 
+## Override Email Headers (`mailer.override_header`)
+
+:::warning
+This is empty by default, use it only if you know what you need it for.
+:::
+
+examples would be:
+
+```ini
+[mailer.override_header]
+Reply-To = test@example.com, test2@example.com
+Content-Type = text/html; charset=utf-8
+In-Reply-To =
+```
+
 ## Incoming Email (`email.incoming`)
 
 - `ENABLED`: **false**: Enable handling of incoming emails.

modules/setting/mailer.go

@@ -18,14 +18,15 @@ import (
 // Mailer represents mail service.
 type Mailer struct {
 	// Mailer
-	Name                 string `ini:"NAME"`
-	From                 string `ini:"FROM"`
-	EnvelopeFrom         string `ini:"ENVELOPE_FROM"`
-	OverrideEnvelopeFrom bool   `ini:"-"`
-	FromName             string `ini:"-"`
-	FromEmail            string `ini:"-"`
-	SendAsPlainText      bool   `ini:"SEND_AS_PLAIN_TEXT"`
-	SubjectPrefix        string `ini:"SUBJECT_PREFIX"`
+	Name                 string              `ini:"NAME"`
+	From                 string              `ini:"FROM"`
+	EnvelopeFrom         string              `ini:"ENVELOPE_FROM"`
+	OverrideEnvelopeFrom bool                `ini:"-"`
+	FromName             string              `ini:"-"`
+	FromEmail            string              `ini:"-"`
+	SendAsPlainText      bool                `ini:"SEND_AS_PLAIN_TEXT"`
+	SubjectPrefix        string              `ini:"SUBJECT_PREFIX"`
+	OverrideHeader       map[string][]string `ini:"-"`
 
 	// SMTP sender
 	Protocol             string `ini:"PROTOCOL"`
@@ -151,6 +152,12 @@ func loadMailerFrom(rootCfg ConfigProvider) {
 		log.Fatal("Unable to map [mailer] section on to MailService. Error: %v", err)
 	}
 
+	overrideHeader := rootCfg.Section("mailer.override_header").Keys()
+	MailService.OverrideHeader = make(map[string][]string)
+	for _, key := range overrideHeader {
+		MailService.OverrideHeader[key.Name()] = key.Strings(",")
+	}
+
 	// Infer SMTPPort if not set
 	if MailService.SMTPPort == "" {
 		switch MailService.Protocol {

services/mailer/mailer.go

@@ -57,7 +57,7 @@ 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)
@@ -76,6 +76,13 @@ func (m *Message) ToMessage() *gomail.Message {
 		msg.AddAlternative("text/html", m.Body)
 	}
 
+	for k, v := range setting.MailService.OverrideHeader {
+		if len(msg.GetHeader(k)) != 0 {
+			log.Debug("Mailer override header '%s' as per config", k)
+		}
+		msg.SetHeader(k, v...)
+	}
+
 	if len(msg.GetHeader("Message-ID")) == 0 {
 		msg.SetHeader("Message-ID", m.generateAutoMessageID())
 	}