diff --git a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php
index e2db955545e40..84dd91b0ef061 100644
--- a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php
+++ b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php
@@ -1555,8 +1555,17 @@ private function addMailerSection(ArrayNodeDefinition $rootNode)
                 ->arrayNode('mailer')
                     ->info('Mailer configuration')
                     ->{!class_exists(FullStack::class) && class_exists(Mailer::class) ? 'canBeDisabled' : 'canBeEnabled'}()
+                    ->validate()
+                        ->ifTrue(function ($v) { return isset($v['dsn']) && \count($v['transports']); })
+                        ->thenInvalid('"dsn" and "transports" cannot be used together.')
+                    ->end()
+                    ->fixXmlConfig('transport')
                     ->children()
-                        ->scalarNode('dsn')->defaultValue('smtp://null')->end()
+                        ->scalarNode('dsn')->defaultNull()->end()
+                        ->arrayNode('transports')
+                            ->useAttributeAsKey('name')
+                            ->prototype('scalar')->end()
+                        ->end()
                         ->arrayNode('envelope')
                             ->info('Mailer Envelope configuration')
                             ->children()
diff --git a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php
index 84bfe6a1224f4..0fa3211e33fb7 100644
--- a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php
+++ b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php
@@ -1977,7 +1977,12 @@ private function registerMailerConfiguration(array $config, ContainerBuilder $co
 
         $loader->load('mailer.xml');
         $loader->load('mailer_transports.xml');
-        $container->getDefinition('mailer.default_transport')->setArgument(0, $config['dsn']);
+        if (!\count($config['transports']) && null === $config['dsn']) {
+            $config['dsn'] = 'smtp://null';
+        }
+        $transports = $config['dsn'] ? ['main' => $config['dsn']] : $config['transports'];
+        $container->getDefinition('mailer.transports')->setArgument(0, $transports);
+        $container->getDefinition('mailer.default_transport')->setArgument(0, current($transports));
 
         $classToServices = [
             SesTransportFactory::class => 'mailer.transport_factory.amazon',
diff --git a/src/Symfony/Bundle/FrameworkBundle/Resources/config/mailer.xml b/src/Symfony/Bundle/FrameworkBundle/Resources/config/mailer.xml
index af71f7b64e22d..6cea7de57203c 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Resources/config/mailer.xml
+++ b/src/Symfony/Bundle/FrameworkBundle/Resources/config/mailer.xml
@@ -6,13 +6,18 @@
 
     <services>
         <service id="mailer.mailer" class="Symfony\Component\Mailer\Mailer">
-            <argument type="service" id="mailer.default_transport" />
+            <argument type="service" id="mailer.transports" />
             <argument type="service" id="messenger.default_bus" on-invalid="ignore" />
             <argument type="service" id="event_dispatcher" on-invalid="ignore" />
         </service>
         <service id="mailer" alias="mailer.mailer" />
         <service id="Symfony\Component\Mailer\MailerInterface" alias="mailer.mailer" />
 
+        <service id="mailer.transports" class="Symfony\Component\Mailer\Transports">
+            <factory service="mailer.transport_factory" method="fromStrings" />
+            <argument type="collection" /> <!-- transports -->
+        </service>
+
         <service id="mailer.transport_factory" class="Symfony\Component\Mailer\Transport">
             <argument type="tagged_iterator" tag="mailer.transport_factory" />
         </service>
@@ -24,7 +29,7 @@
         <service id="Symfony\Component\Mailer\Transport\TransportInterface" alias="mailer.default_transport" />
 
         <service id="mailer.messenger.message_handler" class="Symfony\Component\Mailer\Messenger\MessageHandler">
-            <argument type="service" id="mailer.default_transport" />
+            <argument type="service" id="mailer.transports" />
             <tag name="messenger.message_handler" />
         </service>
 
diff --git a/src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/ConfigurationTest.php b/src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/ConfigurationTest.php
index 39e9271f4e056..efead383b11cf 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/ConfigurationTest.php
+++ b/src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/ConfigurationTest.php
@@ -369,7 +369,8 @@ class_exists(SemaphoreStore::class) && SemaphoreStore::isSupported() ? 'semaphor
                 'scoped_clients' => [],
             ],
             'mailer' => [
-                'dsn' => 'smtp://null',
+                'dsn' => null,
+                'transports' => [],
                 'enabled' => !class_exists(FullStack::class) && class_exists(Mailer::class),
             ],
         ];
diff --git a/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/MailerTest.php b/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/MailerTest.php
index 77e70e9ea3a28..29f8fe972d9b7 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/MailerTest.php
+++ b/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/MailerTest.php
@@ -55,7 +55,7 @@ protected function doSend(SentMessage $message): void
             }
         };
 
-        $mailer = new Mailer($testTransport, null);
+        $mailer = new Mailer($testTransport);
 
         $message = (new Email())
             ->subject('Test subject')
diff --git a/src/Symfony/Component/Mailer/CHANGELOG.md b/src/Symfony/Component/Mailer/CHANGELOG.md
index b1d82f2d89b1a..509e1b657c9ae 100644
--- a/src/Symfony/Component/Mailer/CHANGELOG.md
+++ b/src/Symfony/Component/Mailer/CHANGELOG.md
@@ -4,6 +4,7 @@ CHANGELOG
 4.4.0
 -----
 
+ * added support for multiple transports on a `Mailer` instance
  * [BC BREAK] removed the `auth_mode` DSN option (it is now always determined automatically)
  * STARTTLS cannot be enabled anymore (it is used automatically if TLS is disabled and the server supports STARTTLS)
  * [BC BREAK] Removed the `encryption` DSN option (use `smtps` instead)
diff --git a/src/Symfony/Component/Mailer/Mailer.php b/src/Symfony/Component/Mailer/Mailer.php
index 5c649320e3bbf..8aa357de4ca7e 100644
--- a/src/Symfony/Component/Mailer/Mailer.php
+++ b/src/Symfony/Component/Mailer/Mailer.php
@@ -22,7 +22,7 @@
 /**
  * @author Fabien Potencier <fabien@symfony.com>
  */
-class Mailer implements MailerInterface
+final class Mailer implements MailerInterface
 {
     private $transport;
     private $bus;
diff --git a/src/Symfony/Component/Mailer/Transport.php b/src/Symfony/Component/Mailer/Transport.php
index 6617f241209db..3949fa78c508e 100644
--- a/src/Symfony/Component/Mailer/Transport.php
+++ b/src/Symfony/Component/Mailer/Transport.php
@@ -27,6 +27,7 @@
 use Symfony\Component\Mailer\Transport\Smtp\EsmtpTransportFactory;
 use Symfony\Component\Mailer\Transport\TransportFactoryInterface;
 use Symfony\Component\Mailer\Transport\TransportInterface;
+use Symfony\Component\Mailer\Transport\Transports;
 use Symfony\Contracts\EventDispatcher\EventDispatcherInterface;
 use Symfony\Contracts\HttpClient\HttpClientInterface;
 
@@ -54,6 +55,13 @@ public static function fromDsn(string $dsn, EventDispatcherInterface $dispatcher
         return $factory->fromString($dsn);
     }
 
+    public static function fromDsns(array $dsns, EventDispatcherInterface $dispatcher = null, HttpClientInterface $client = null, LoggerInterface $logger = null): TransportInterface
+    {
+        $factory = new self(iterator_to_array(self::getDefaultFactories($dispatcher, $client, $logger)));
+
+        return $factory->fromStrings($dsns);
+    }
+
     /**
      * @param TransportFactoryInterface[] $factories
      */
@@ -62,6 +70,16 @@ public function __construct(iterable $factories)
         $this->factories = $factories;
     }
 
+    public function fromStrings(array $dsns): Transports
+    {
+        $transports = [];
+        foreach ($dsns as $name => $dsn) {
+            $transports[$name] = $this->fromString($dsn);
+        }
+
+        return new Transports($transports);
+    }
+
     public function fromString(string $dsn): TransportInterface
     {
         $dsns = preg_split('/\s++\|\|\s++/', $dsn);
diff --git a/src/Symfony/Component/Mailer/Transport/Transports.php b/src/Symfony/Component/Mailer/Transport/Transports.php
new file mode 100644
index 0000000000000..a7d52e545e03e
--- /dev/null
+++ b/src/Symfony/Component/Mailer/Transport/Transports.php
@@ -0,0 +1,69 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mailer\Transport;
+
+use Symfony\Component\Mailer\Exception\InvalidArgumentException;
+use Symfony\Component\Mailer\Exception\LogicException;
+use Symfony\Component\Mailer\SentMessage;
+use Symfony\Component\Mailer\SmtpEnvelope;
+use Symfony\Component\Mime\Message;
+use Symfony\Component\Mime\RawMessage;
+
+/**
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+class Transports implements TransportInterface
+{
+    private $transports;
+    private $default;
+
+    /**
+     * @param TransportInterface[] $transports
+     */
+    public function __construct(iterable $transports)
+    {
+        $this->transports = [];
+        foreach ($transports as $name => $transport) {
+            if (null === $this->default) {
+                $this->default = $transport;
+            }
+            $this->transports[$name] = $transport;
+        }
+
+        if (!$this->transports) {
+            throw new LogicException(sprintf('"%s" must have at least one transport configured.', __CLASS__));
+        }
+    }
+
+    public function send(RawMessage $message, SmtpEnvelope $envelope = null): ?SentMessage
+    {
+        /** @var Message $message */
+        if (RawMessage::class === \get_class($message) || !$message->getHeaders()->has('X-Transport')) {
+            return $this->default->send($message, $envelope);
+        }
+
+        $headers = $message->getHeaders();
+        $transport = $headers->get('X-Transport');
+        $headers->remove('X-Transport');
+
+        if (!isset($this->transports[$transport])) {
+            throw new InvalidArgumentException(sprintf('The "%s" transport does not exist.', $transport));
+        }
+
+        return $this->transports[$transport]->send($message, $envelope);
+    }
+
+    public function __toString(): string
+    {
+        return 'all';
+    }
+}

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