[Mail] Fixed: mailer transports main is ignored (#18)

* [Mail] Fixed: mailer transports main is ignored, fixes pimcore#15824

* fixed stan

Author: brusch

lib/Mail/Mailer.php

@@ -20,6 +20,7 @@
 use Pimcore\Mail\Plugins\RedirectingPlugin;
 use Symfony\Component\Mailer\Envelope;
 use Symfony\Component\Mailer\MailerInterface;
+use Symfony\Component\Mime\Message;
 use Symfony\Component\Mime\RawMessage;
 
 class Mailer implements MailerInterface
@@ -40,6 +41,10 @@ public function send(RawMessage $message, Envelope $envelope = null): void
             $this->redirectPlugin->beforeSendPerformed($message);
         }
 
+        if($message instanceof Message && !$message->getHeaders()->has('X-Transport')) {
+            $message->getHeaders()->addTextHeader('X-Transport', 'main');
+        }
+
         $this->mailer->send($message, $envelope);
 
         if ($message instanceof Mail) {