feat(mailer): set localname to hostname

Some email providers does not accept localhost
in the HELO message. So we set it to hostname
instead. Our hope is that should be enough for
those providers who does not accept localhost.

closes #7223

pkg/services/notifications/mailer.go

@@ -101,6 +101,7 @@ func createDialer() (*gomail.Dialer, error) {
 
 	d := gomail.NewDialer(host, iPort, setting.Smtp.User, setting.Smtp.Password)
 	d.TLSConfig = tlsconfig
+	d.LocalName = setting.InstanceName
 	return d, nil
 }