Don't lookup mail server when using sendmail (#22300) (#22383)

Fix #22287
backport #22300

modules/setting/mailer.go

@@ -179,14 +179,25 @@ func newMailService() {
 
 	// we want to warn if users use SMTP on a non-local IP;
 	// we might as well take the opportunity to check that it has an IP at all
-	ips := tryResolveAddr(MailService.SMTPAddr)
-	if MailService.Protocol == "smtp" {
-		for _, ip := range ips {
-			if !ip.IsLoopback() {
-				log.Warn("connecting over insecure SMTP protocol to non-local address is not recommended")
-				break
+	// This check is not needed for sendmail
+	switch MailService.Protocol {
+	case "sendmail":
+		var err error
+		MailService.SendmailArgs, err = shellquote.Split(sec.Key("SENDMAIL_ARGS").String())
+		if err != nil {
+			log.Error("Failed to parse Sendmail args: '%s' with error %v", sec.Key("SENDMAIL_ARGS").String(), err)
+		}
+	case "smtp", "smtps", "smtp+starttls", "smtp+unix":
+		ips := tryResolveAddr(MailService.SMTPAddr)
+		if MailService.Protocol == "smtp" {
+			for _, ip := range ips {
+				if !ip.IsLoopback() {
+					log.Warn("connecting over insecure SMTP protocol to non-local address is not recommended")
+					break
+				}
 			}
 		}
+	case "dummy": // just mention and do nothing
 	}
 
 	if MailService.From != "" {
@@ -215,14 +226,6 @@ func newMailService() {
 		MailService.EnvelopeFrom = parsed.Address
 	}
 
-	if MailService.Protocol == "sendmail" {
-		var err error
-		MailService.SendmailArgs, err = shellquote.Split(sec.Key("SENDMAIL_ARGS").String())
-		if err != nil {
-			log.Error("Failed to parse Sendmail args: %s with error %v", CustomConf, err)
-		}
-	}
-
 	log.Info("Mail Service Enabled")
 }