pkg/mailer: support plaintext alt for HTML emails (#5568)

* Added option to use plain text alt to HTML emails. Should make the messages friendlier for spam filters.

* Check that plaintext conversion worked before adding the HTML alt

* Add description of ADD_PLAIN_TEXT_ALT to app.ini

* Added comment clarifying html AddAlternative

conf/app.ini

@@ -250,6 +250,8 @@ USER =
 PASSWD =
 ; Use text/plain as format of content
 USE_PLAIN_TEXT = false
+; If sending html emails, then also attach a plaintext alternative to the MIME message, to support older mail clients and make spam filters happier.
+ADD_PLAIN_TEXT_ALT = false
 
 [cache]
 ; Either "memory", "redis", or "memcache", default is "memory"

pkg/mailer/mailer.go

@@ -39,16 +39,24 @@ func NewMessageFrom(to []string, from, subject, htmlBody string) *Message {
 
 	contentType := "text/html"
 	body := htmlBody
-	if setting.MailService.UsePlainText {
+	switchedToPlaintext := false
+	if setting.MailService.UsePlainText || setting.MailService.AddPlainTextAlt {
 		plainBody, err := html2text.FromString(htmlBody)
 		if err != nil {
 			log.Error(2, "html2text.FromString: %v", err)
 		} else {
 			contentType = "text/plain"
 			body = plainBody
+			switchedToPlaintext = true
 		}
 	}
 	msg.SetBody(contentType, body)
+	if switchedToPlaintext && setting.MailService.AddPlainTextAlt && !setting.MailService.UsePlainText {
+		// The AddAlternative method name is confusing - adding html as an "alternative" will actually cause mail 
+		// clients to show it as first priority, and the text "main body" is the 2nd priority fallback.
+		// See: https://godoc.org/gopkg.in/gomail.v2#Message.AddAlternative
+		msg.AddAlternative("text/html", htmlBody)
+	}
 	return &Message{
 		Message:     msg,
 		confirmChan: make(chan struct{}),

pkg/setting/setting.go

@@ -861,6 +861,7 @@ type Mailer struct {
 	UseCertificate    bool
 	CertFile, KeyFile string
 	UsePlainText      bool
+	AddPlainTextAlt   bool
 }
 
 var (
@@ -888,6 +889,7 @@ func newMailService() {
 		CertFile:        sec.Key("CERT_FILE").String(),
 		KeyFile:         sec.Key("KEY_FILE").String(),
 		UsePlainText:    sec.Key("USE_PLAIN_TEXT").MustBool(),
+		AddPlainTextAlt: sec.Key("ADD_PLAIN_TEXT_ALT").MustBool(),
 	}
 	MailService.From = sec.Key("FROM").MustString(MailService.User)