feat: send mailings over MailJet smtp in dedicated worker process

.env

@@ -38,6 +38,9 @@ MESSENGER_TRANSPORT_DSN=doctrine://default?auto_setup=0
 
 ###> symfony/mailer ###
 MAILER_DSN=null://null
+# Has to resolve in every environment or the container does not compile, see the
+# mailjet section below for the production value
+MAILING_MAILER_DSN=null://null
 ###< symfony/mailer ###
 
 APP_BASE_URI=https://myep-team.ddev.site
@@ -68,8 +71,18 @@ MYEP_OAUTH2_URL_RESOURCE_OWNER_DETAILS=https://my.ep-reisen.de/api/userinfo
 MYEP_OAUTH2_SCOPES=email,id,roles,profile
 
 ###> symfony/mailjet-mailer ###
-# MAILER_DSN=mailjet+api://PUBLIC_KEY:[email protected]
-# #MAILER_DSN=mailjet+smtp://PUBLIC_KEY:[email protected]
+# Bulk teamer mailings only, everything else goes out over MAILER_DSN. Set the real
+# credentials in .env.local - the sending domain has to be verified in Mailjet (SPF,
+# DKIM, DMARC) or the mail is delivered but lands in spam.
+#
+# Use smtp. The api transport does not send the message we built: it takes the parts as
+# json fields and has Mailjet assemble the mail, which drops the embedded logo into an
+# ordinary attachment. Over smtp our own MIME goes out unchanged and the mail arrives as
+# it was rendered. Deliverability is identical either way, it is the same account, the
+# same ip pool and the same dkim signature. The api is only worth revisiting if the host
+# ever blocks outbound port 587, since it talks https instead.
+# MAILING_MAILER_DSN=mailjet+smtp://PUBLIC_KEY:[email protected]
+# MAILING_MAILER_DSN=mailjet+api://PUBLIC_KEY:PRIVATE_KEY@default
 ###< symfony/mailjet-mailer ###
 
 BIN_GS=/usr/bin/gs

config/packages/mailer.yaml

@@ -1,3 +1,8 @@
 framework:
     mailer:
-        dsn: '%env(MAILER_DSN)%'
+        # "main" has to stay first: it is the transport every mail without an explicit
+        # X-Transport header goes out over, which is all of them apart from the teamer
+        # mailing. Reordering this quietly moves the whole application to Mailjet.
+        transports:
+            main: '%env(MAILER_DSN)%'
+            mailing: '%env(MAILING_MAILER_DSN)%'

config/packages/messenger.yaml

@@ -12,6 +12,29 @@ framework:
                 retry_strategy:
                     max_retries: 3
                     multiplier: 2
+            # Bulk teamer mailings, kept off "async" so that a mailing of several hundred
+            # recipients cannot delay a password reset. Its own cron worker consumes this
+            # queue and nothing else; the two never see each other's messages, because the
+            # doctrine transport filters on the queue_name column. See docs/operations.md
+            # for both cron entries.
+            #
+            # The retry delays are spelled out because the default is one second: with a
+            # multiplier alone, all three retries of a mail are spent within seconds, so a
+            # short hiccup at the mail provider would drop a whole mailing into "failed" at
+            # once - the very thing sending one message per recipient is meant to avoid.
+            # A minute, three and nine outlive any realistic blip and span several runs of
+            # a worker that only lives for a few minutes at a time.
+            mailing:
+                dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
+                options:
+                    queue_name: mailing
+                    use_notify: true
+                    check_delayed_interval: 60000
+                retry_strategy:
+                    max_retries: 3
+                    delay: 60000
+                    multiplier: 3
+                    max_delay: 900000
             failed: 'doctrine://default?queue_name=failed'
             sync: 'sync://'
 
@@ -20,6 +43,9 @@ framework:
             Symfony\Component\Notifier\Message\ChatMessage: async
             Symfony\Component\Notifier\Message\SmsMessage: async
 
+            # only the fan-out, the mails it produces carry an X-Bus-Transport header
+            App\Message\SendTeamerMailing: mailing
+
             # Route your messages to the transports
             # 'App\Message\YourMessage': async
 
@@ -30,3 +56,13 @@ when@dev:
                 Symfony\Component\Mailer\Messenger\SendEmailMessage: sync
                 Symfony\Component\Notifier\Message\ChatMessage: sync
                 Symfony\Component\Notifier\Message\SmsMessage: sync
+                App\Message\SendTeamerMailing: sync
+
+when@test:
+    framework:
+        messenger:
+            routing:
+                Symfony\Component\Mailer\Messenger\SendEmailMessage: sync
+                Symfony\Component\Notifier\Message\ChatMessage: sync
+                Symfony\Component\Notifier\Message\SmsMessage: sync
+                App\Message\SendTeamerMailing: sync

config/packages/zenstruck_schedule.yaml

@@ -3,13 +3,13 @@ zenstruck_schedule:
 
   mailer:
     service: mailer
-    default_to: [email protected]
-    default_from: info@ep-reisen.de
+    default_to: [email protected]
+    default_from: team@ep-reisen.de
     subject_prefix: "[MyE&P-Team]"
 
   schedule_extensions:
     email_on_failure:
-      to: [email protected]
+      to: [email protected]
 
   tasks:
     - task: app:bpn-import

config/services.yaml

@@ -9,6 +9,13 @@ parameters:
 
     teamer_inactive_period: '-2 years'
 
+    # Messenger transport the mails of a teamer mailing are queued on. An X-Bus-Transport
+    # header wins over the routing in messenger.yaml (see SendersLocator::getSenders),
+    # so this is what keeps a mailing off the shared queue - and what has to be switched
+    # back to "sync" further down, or a mailing would need a running worker to arrive
+    # in dev and test while every other mail is sent right away.
+    mailing_bus_transport: 'mailing'
+
     # Dates (MM-DD) on which teamers must re-confirm their personal data.
     personal_data_check_deadlines:
         - '04-01'
@@ -51,6 +58,7 @@ services:
             $xmlExport: '@xml_export.storage'
             $xmlDump: '@xml_dump.storage'
             $teamerInactivePeriod: '%teamer_inactive_period%'
+            $mailingBusTransport: '%mailing_bus_transport%'
 
     App\:
         resource: '../src/'
@@ -243,3 +251,15 @@ services:
                 myep_oauth2_url_access_token: '%env(MYEP_OAUTH2_URL_ACCESS_TOKEN)%'
                 myep_oauth2_url_resource_owner_details: '%env(MYEP_OAUTH2_URL_RESOURCE_OWNER_DETAILS)%'
                 myep_oauth2_scopes: '%env(csv:MYEP_OAUTH2_SCOPES)%'
+
+# The mails of a mailing are sent right away here, like every other mail, instead of
+# waiting for a worker on the dedicated queue. The mailer transport is deliberately not
+# switched: a mailing still goes out over MAILING_MAILER_DSN, so the path being exercised
+# locally is the one that runs in production.
+when@dev:
+    parameters:
+        mailing_bus_transport: 'sync'
+
+when@test:
+    parameters:
+        mailing_bus_transport: 'sync'

src/Controller/Admin/Teamer/MailingController.php

@@ -5,6 +5,7 @@ namespace App\Controller\Admin\Teamer;
 use App\Controller\Traits\ReturnUrlTrait;
 use App\Form\TeamerMailingType;
 use App\Htmx\HxRedirectResponse;
+use App\Message\SendTeamerMailing;
 use App\Service\Common\TeamerFilterHandler;
 use App\Service\Teamer\TeamerMailingDraftHandler;
 use App\Service\Teamer\TeamerMailingService;
@@ -12,6 +13,7 @@ use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
 use Symfony\Component\Form\FormInterface;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\Messenger\MessageBusInterface;
 use Symfony\Component\Routing\Attribute\Route;
 use Symfony\Component\Security\Http\Attribute\IsGranted;
 
@@ -23,6 +25,7 @@ class MailingController extends AbstractController
         private readonly TeamerFilterHandler $filterHandler,
         private readonly TeamerMailingService $mailingService,
         private readonly TeamerMailingDraftHandler $draftHandler,
+        private readonly MessageBusInterface $messageBus,
     ) {
     }
 
@@ -114,12 +117,25 @@ class MailingController extends AbstractController
         }
 
         $recipients = $this->mailingService->resolveRecipients($this->filterHandler->getFilterSettings());
-        $count = $this->mailingService->send($form->getData(), $recipients);
+        $mailingDto = $form->getData();
 
-        // it went out, the next mailing starts on a blank page
+        // the recipients travel with the message, so the mailing reaches exactly the
+        // people the confirmation modal counted even though it is sent from a worker
+        $this->messageBus->dispatch(SendTeamerMailing::fromRecipients(
+            (string) $mailingDto->getSubject(),
+            (string) $mailingDto->getMessage(),
+            $recipients
+        ));
+
+        // it is on its way, the next mailing starts on a blank page
         $this->draftHandler->resetDraft();
 
-        $this->addFlash('success', sprintf('Die Mail wurde an %d Teamer:innen gesendet', $count));
+        // deliberately not "wurde gesendet": at this point nothing has been handed to a
+        // mail server yet, and saying otherwise would make a failed mailing look fine
+        $this->addFlash('success', sprintf(
+            'Die Mail wird an %d Teamer:innen gesendet',
+            $recipients->getEligibleCount()
+        ));
 
         return new HxRedirectResponse($this->generateUrl('app_administrative_teamer_index'));
     }

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');
+        }
+    }
+}

src/Message/SendTeamerMailing.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace App\Message;
+
+use App\Model\TeamerMailingRecipientsDto;
+
+/**
+ * One composed mailing, on its way to the recipients the admin confirmed.
+ *
+ * The recipients travel as a snapshot of plain scalars rather than as a filter to run
+ * later: re-running the filter in the worker could reach a different set of people than
+ * the one the confirmation modal showed, and the filter itself carries hydrated entities
+ * that have no business being serialized into a queue row. Plain arrays also survive a
+ * deploy that changes TeamerMailingRecipient while messages are still queued.
+ */
+class SendTeamerMailing
+{
+    /**
+     * @param array<int, array{email: string, firstName: string, lastName: string, fullName: string}> $recipients
+     */
+    public function __construct(
+        private readonly string $subject,
+        private readonly string $message,
+        private readonly array $recipients,
+    ) {
+    }
+
+    public static function fromRecipients(string $subject, string $message, TeamerMailingRecipientsDto $recipients): self
+    {
+        $snapshot = [];
+
+        foreach ($recipients->getEligible() as $recipient) {
+            // resolveRecipients() already guaranteed the address, this only narrows the type
+            if (null === $email = $recipient->getEmail()) {
+                continue;
+            }
+
+            $snapshot[] = [
+                'email' => $email,
+                'firstName' => $recipient->getFirstName(),
+                'lastName' => $recipient->getLastName(),
+                'fullName' => $recipient->getFullName(),
+            ];
+        }
+
+        return new self($subject, $message, $snapshot);
+    }
+
+    public function getSubject(): string
+    {
+        return $this->subject;
+    }
+
+    public function getMessage(): string
+    {
+        return $this->message;
+    }
+
+    /**
+     * @return array<int, array{email: string, firstName: string, lastName: string, fullName: string}>
+     */
+    public function getRecipients(): array
+    {
+        return $this->recipients;
+    }
+
+    public function getRecipientCount(): int
+    {
+        return count($this->recipients);
+    }
+}

src/MessageHandler/SendTeamerMailingHandler.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace App\MessageHandler;
+
+use App\Message\SendTeamerMailing;
+use App\Service\Teamer\TeamerMailingService;
+use Symfony\Component\Messenger\Attribute\AsMessageHandler;
+
+/**
+ * Fans a confirmed mailing out into one queued mail per recipient.
+ *
+ * The request that confirmed the mailing only enqueues this single message, so it answers
+ * in constant time no matter how many teamers the filter matched, and a mailing is either
+ * enqueued whole or not at all. Every mail this produces is queued separately, so a
+ * recipient whose address bounces is retried on its own instead of dragging the rest of
+ * the mailing through a retry with it.
+ */
+#[AsMessageHandler]
+class SendTeamerMailingHandler
+{
+    public function __construct(
+        private readonly TeamerMailingService $mailingService,
+    ) {
+    }
+
+    public function __invoke(SendTeamerMailing $mailing): void
+    {
+        $this->mailingService->sendMailing($mailing);
+    }
+}

src/Service/Teamer/TeamerMailingService.php

@@ -4,6 +4,7 @@ namespace App\Service\Teamer;
 
 use App\Email\Mailer;
 use App\Entity\User;
+use App\Message\SendTeamerMailing;
 use App\Model\TeamerFilterDto;
 use App\Model\TeamerMailingDto;
 use App\Model\TeamerMailingRecipient;
@@ -26,10 +27,20 @@ class TeamerMailingService
     private const TEMPLATE = 'email/teamer_mailing.html.twig';
     private const PREVIEW_SUBJECT_PREFIX = '[Vorschau] ';
 
+    /**
+     * The mailer transport a mailing goes out over: Mailjet, rather than the webhoster's
+     * relay that carries every other mail, which would risk the hosting account over a few
+     * hundred recipients. Configured in mailer.yaml.
+     */
+    private const TRANSPORT = 'mailing';
+
     public function __construct(
         private readonly TeamerRepository $teamerRepository,
         private readonly Mailer $mailer,
         private readonly LoggerInterface $logger,
+        // messenger transport, "mailing" in production and "sync" in dev and test,
+        // see the parameter of the same name in services.yaml
+        private readonly string $mailingBusTransport,
     ) {
     }
 
@@ -68,32 +79,35 @@ class TeamerMailingService
     }
 
     /**
-     * Send the mailing to everyone eligible and return how many mails went out.
+     * Hand every recipient of a confirmed mailing to the mailer.
+     *
+     * This runs in the worker, not in the request that confirmed the mailing, so it
+     * reports nothing back: each mail becomes a queued message of its own and whether it
+     * reaches anyone is decided long after this method returns. A caller that wants to
+     * know how many mails were actually delivered has to ask the mail provider.
      */
-    public function send(TeamerMailingDto $mailingDto, TeamerMailingRecipientsDto $recipients): int
+    public function sendMailing(SendTeamerMailing $mailing): void
     {
-        foreach ($recipients->getEligible() as $recipient) {
-            // resolveRecipients() already guaranteed the address, this only narrows the type
-            if (null === $email = $recipient->getEmail()) {
-                continue;
-            }
-
-            $values = $this->placeholderValuesFor($recipient);
+        foreach ($mailing->getRecipients() as $recipient) {
+            $values = $this->buildValues(
+                $recipient['firstName'],
+                $recipient['lastName'],
+                $recipient['fullName']
+            );
 
             $this->sendTo(
-                $email,
-                $this->render($mailingDto->getSubject(), $values),
-                $this->render($mailingDto->getMessage(), $values)
+                $recipient['email'],
+                $this->render($mailing->getSubject(), $values),
+                $this->render($mailing->getMessage(), $values),
+                self::TRANSPORT,
+                $this->mailingBusTransport
             );
         }
 
         $this->logger->info('Send teamer mailing', [
-            'subject' => $mailingDto->getSubject(),
-            'recipients' => $recipients->getEligibleCount(),
-            'skipped' => $recipients->getSkippedCount(),
+            'subject' => $mailing->getSubject(),
+            'recipients' => $mailing->getRecipientCount(),
         ]);
-
-        return $recipients->getEligibleCount();
     }
 
     /**
@@ -173,14 +187,26 @@ class TeamerMailingService
         ];
     }
 
-    private function sendTo(string $email, string $subject, string $message): void
-    {
+    /**
+     * The preview leaves the transports unset on purpose: it goes out over the same relay
+     * and the same queue as the rest of the application, so it arrives while the admin is
+     * still looking at the compose page.
+     */
+    private function sendTo(
+        string $email,
+        string $subject,
+        string $message,
+        ?string $transport = null,
+        ?string $busTransport = null,
+    ): void {
         $this->mailer->createAndSendEmail([
             'message' => $message,
         ], [
             'to' => $email,
             'subject' => $subject,
             'template' => self::TEMPLATE,
+            'transport' => $transport,
+            'bus_transport' => $busTransport,
         ]);
     }
 }

tests/Email/RemoveBusTransportHeaderListenerTest.php

@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Email;
+
+use App\Email\RemoveBusTransportHeaderListener;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Mailer\Envelope;
+use Symfony\Component\Mailer\Event\MessageEvent;
+use Symfony\Component\Mime\Address;
+use Symfony\Component\Mime\Email;
+
+class RemoveBusTransportHeaderListenerTest extends TestCase
+{
+    public function testTheHeaderIsGoneFromTheDeliveredMail(): void
+    {
+        $email = $this->createEmail();
+
+        (new RemoveBusTransportHeaderListener())($this->createEvent($email, queued: false));
+
+        $this->assertFalse($email->getHeaders()->has('X-Bus-Transport'));
+    }
+
+    /**
+     * Symfony reads the header off the queued message to pick the messenger transport, so
+     * removing it here would send the mailing over the shared queue after all.
+     */
+    public function testTheHeaderSurvivesQueueing(): void
+    {
+        $email = $this->createEmail();
+
+        (new RemoveBusTransportHeaderListener())($this->createEvent($email, queued: true));
+
+        $this->assertTrue($email->getHeaders()->has('X-Bus-Transport'));
+    }
+
+    private function createEmail(): Email
+    {
+        $email = (new Email())
+            ->from('[email protected]')
+            ->to('[email protected]')
+            ->subject('Betreff')
+            ->text('Nachricht')
+        ;
+
+        $email->getHeaders()->addTextHeader('X-Bus-Transport', 'mailing');
+
+        return $email;
+    }
+
+    private function createEvent(Email $email, bool $queued): MessageEvent
+    {
+        return new MessageEvent(
+            $email,
+            new Envelope(new Address('[email protected]'), [new Address('[email protected]')]),
+            'mailing',
+            $queued
+        );
+    }
+}

tests/MessageHandler/SendTeamerMailingHandlerTest.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\MessageHandler;
+
+use App\Message\SendTeamerMailing;
+use App\MessageHandler\SendTeamerMailingHandler;
+use App\Service\Teamer\TeamerMailingService;
+use PHPUnit\Framework\TestCase;
+
+class SendTeamerMailingHandlerTest extends TestCase
+{
+    public function testTheMailingIsHandedToTheServiceUnchanged(): void
+    {
+        $mailing = new SendTeamerMailing('Betreff', 'Nachricht', [
+            ['email' => '[email protected]', 'firstName' => 'Anna', 'lastName' => 'Berg', 'fullName' => 'Anna Berg'],
+        ]);
+
+        $service = $this->createMock(TeamerMailingService::class);
+        $service
+            ->expects($this->once())
+            ->method('sendMailing')
+            ->with($this->identicalTo($mailing))
+        ;
+
+        (new SendTeamerMailingHandler($service))($mailing);
+    }
+}

tests/Service/Teamer/TeamerMailingServiceTest.php

@@ -7,9 +7,11 @@ namespace App\Tests\Service\Teamer;
 use App\Email\Mailer;
 use App\Entity\Teamer;
 use App\Entity\User;
+use App\Message\SendTeamerMailing;
 use App\Model\TeamerFilterDto;
 use App\Model\TeamerMailingDto;
 use App\Model\TeamerMailingRecipient;
+use App\Model\TeamerMailingRecipientsDto;
 use App\Repository\TeamerRepository;
 use App\Service\Teamer\TeamerMailingService;
 use PHPUnit\Framework\MockObject\MockObject;
@@ -32,6 +34,7 @@ class TeamerMailingServiceTest extends TestCase
             $this->teamerRepository,
             $this->mailer,
             $this->createMock(LoggerInterface::class),
+            'mailing',
         );
     }
 
@@ -101,16 +104,8 @@ class TeamerMailingServiceTest extends TestCase
         $this->assertSame(5, $recipients->getTotal());
     }
 
-    public function testSendPersonalisesSubjectAndMessagePerRecipient(): void
+    public function testSendMailingPersonalisesSubjectAndMessagePerRecipient(): void
     {
-        $this->teamerRepository
-            ->method('getMailingRecipients')
-            ->willReturn([
-                $this->createRecipient('Anna', 'Berg', '[email protected]'),
-                $this->createRecipient('Bea', 'Ohm', '[email protected]'),
-            ])
-        ;
-
         $sent = [];
         $this->mailer
             ->expects($this->exactly(2))
@@ -120,19 +115,87 @@ class TeamerMailingServiceTest extends TestCase
             })
         ;
 
+        $this->service->sendMailing($this->createMailing(
+            'Hallo {{vorname}}',
+            'Servus {{name}}',
+            $this->createRecipient('Anna', 'Berg', '[email protected]'),
+            $this->createRecipient('Bea', 'Ohm', '[email protected]')
+        ));
+
+        $this->assertSame([
+            ['[email protected]', 'Hallo Anna', 'Servus Anna Berg'],
+            ['[email protected]', 'Hallo Bea', 'Servus Bea Ohm'],
+        ], $sent);
+    }
+
+    /**
+     * Without both transports a mailing silently leaves over the webhoster's relay on the
+     * shared queue - it still arrives, so nothing about it looks broken, which is exactly
+     * why it is pinned here.
+     */
+    public function testSendMailingGoesOutOverTheMailingTransports(): void
+    {
+        $this->mailer
+            ->expects($this->once())
+            ->method('createAndSendEmail')
+            ->willReturnCallback(function (array $context, array $options): void {
+                $this->assertSame('mailing', $options['transport']);
+                $this->assertSame('mailing', $options['bus_transport']);
+            })
+        ;
+
+        $this->service->sendMailing($this->createMailing(
+            'Betreff',
+            'Nachricht',
+            $this->createRecipient('Anna', 'Berg', '[email protected]')
+        ));
+    }
+
+    public function testSendMailingWithoutRecipientsSendsNothing(): void
+    {
+        $this->mailer
+            ->expects($this->never())
+            ->method('createAndSendEmail')
+        ;
+
+        $this->service->sendMailing($this->createMailing('Betreff', 'Nachricht'));
+    }
+
+    /**
+     * A preview has to stay on the default relay and the default queue, or composing a
+     * mailing would wait on the same worker the mailing itself is queued behind.
+     */
+    public function testPreviewLeavesTheTransportsAlone(): void
+    {
+        $this->mailer
+            ->expects($this->once())
+            ->method('createAndSendEmail')
+            ->willReturnCallback(function (array $context, array $options): void {
+                $this->assertNull($options['transport']);
+                $this->assertNull($options['bus_transport']);
+            })
+        ;
+
         $mailingDto = (new TeamerMailingDto())
             ->setSubject('Hallo {{vorname}}')
             ->setMessage('Servus {{name}}')
         ;
 
-        $recipients = $this->service->resolveRecipients(new TeamerFilterDto());
-        $count = $this->service->send($mailingDto, $recipients);
+        $this->service->sendPreview($mailingDto, (new User())->setEmail('[email protected]'));
+    }
 
-        $this->assertSame(2, $count);
-        $this->assertSame([
-            ['[email protected]', 'Hallo Anna', 'Servus Anna Berg'],
-            ['[email protected]', 'Hallo Bea', 'Servus Bea Ohm'],
-        ], $sent);
+    public function testMailingSnapshotSkipsRecipientsWithoutAnAddress(): void
+    {
+        $recipients = (new TeamerMailingRecipientsDto())
+            ->addEligible($this->createRecipient('Anna', 'Berg', '[email protected]'))
+            ->addEligible($this->createRecipient('Dana', 'Elf', null))
+        ;
+
+        $mailing = SendTeamerMailing::fromRecipients('Betreff', 'Nachricht', $recipients);
+
+        $this->assertSame(1, $mailing->getRecipientCount());
+        $this->assertSame('[email protected]', $mailing->getRecipients()[0]['email']);
+        $this->assertSame('Anna Berg', $mailing->getRecipients()[0]['fullName']);
     }
 
     public function testSendPreviewUsesAdminOwnNameAndMarksTheSubject(): void
@@ -191,6 +254,17 @@ class TeamerMailingServiceTest extends TestCase
         $this->assertSame('rita.kern', $values[TeamerMailingService::PLACEHOLDER_FULL_NAME]);
     }
 
+    private function createMailing(string $subject, string $message, TeamerMailingRecipient ...$recipients): SendTeamerMailing
+    {
+        $dto = new TeamerMailingRecipientsDto();
+
+        foreach ($recipients as $recipient) {
+            $dto->addEligible($recipient);
+        }
+
+        return SendTeamerMailing::fromRecipients($subject, $message, $dto);
+    }
+
     private function createRecipient(
         string $firstName,
         string $lastName,