feat: send mailings over MailJet smtp in dedicated worker process

src/Email/Mailer.php

@@ -44,6 +44,20 @@ class Mailer
             ->context($context)
         ;
 
+        // Both headers are read and removed further down the stack: X-Transport by
+        // Mailer\Transport\Transports when the mail is handed to a transport, and
+        // X-Bus-Transport by Mailer\EventListener\MessengerTransportListener while the
+        // mail is queued. Without them a mail takes the first mailer transport and the
+        // routing configured for SendEmailMessage, which is what every caller but the
+        // teamer mailing wants.
+        if (null !== $config['transport']) {
+            $email->getHeaders()->addTextHeader('X-Transport', $config['transport']);
+        }
+
+        if (null !== $config['bus_transport']) {
+            $email->getHeaders()->addTextHeader('X-Bus-Transport', $config['bus_transport']);
+        }
+
         foreach ($config['attachments'] as $attachment) {
             /* @var EmailAttachmentInterface $attachment */
             $attachment->attachTo($email);
@@ -83,6 +97,8 @@ class Mailer
                 'to' => $this->defaults['to'],
                 'subject_parameters' => [],
                 'attachments' => [],
+                'transport' => null,
+                'bus_transport' => null,
             ])
             ->setRequired([
                 'template',
@@ -93,6 +109,8 @@ class Mailer
             ->setAllowedTypes('subject', 'string')
             ->setAllowedTypes('subject_parameters', 'array')
             ->setAllowedTypes('attachments', 'array')
+            ->setAllowedTypes('transport', ['null', 'string'])
+            ->setAllowedTypes('bus_transport', ['null', 'string'])
         ;
 
         return $resolver->resolve($options);

src/Email/RemoveBusTransportHeaderListener.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace App\Email;
+
+use Symfony\Component\EventDispatcher\Attribute\AsEventListener;
+use Symfony\Component\Mailer\Event\MessageEvent;
+use Symfony\Component\Mime\Message;
+
+/**
+ * Keeps the X-Bus-Transport header out of the mail that is actually delivered.
+ *
+ * Symfony's own listener removes the header while queueing, but it is handed a clone of
+ * the message and the original is what goes onto the bus (see Mailer::send(), which says
+ * so in as many words). The routing works either way - the stamp is read off that clone -
+ * but without this the header travels all the way to the recipient, and with the Mailjet
+ * API transport it is forwarded as a custom header on top, telling everyone which queue
+ * we sort our mail into.
+ *
+ * Only the delivering pass is of interest here: while the mail is queued the header still
+ * has to be there for Symfony to route on.
+ */
+#[AsEventListener(event: MessageEvent::class)]
+class RemoveBusTransportHeaderListener
+{
+    public function __invoke(MessageEvent $event): void
+    {
+        if (true === $event->isQueued()) {
+            return;
+        }
+
+        $message = $event->getMessage();
+
+        if ($message instanceof Message) {
+            $message->getHeaders()->remove('X-Bus-Transport');
+        }
+    }
+}