Add Custom Transport Factories

This might be useful for very custom use cases where we need to extract some data or do something before being able to create the actual transport.

Author: AlbertMorenoDEV

mailer.rst

@@ -215,6 +215,55 @@ As with the failover transport, round-robin retries deliveries until
 a transport succeeds (or all fail). In contrast to the failover transport,
 it *spreads* the load across all its transports.
 
+Custom Transport Factories
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There is a way to easily create your own custom transport factory in case you need to do something special creating the actual transport.
+
+The new factory needs to implement `Symfony\Component\Mailer\Transport\TransportFactoryInterface`. To remove some boilerplate you can even extend from `Symfony\Component\Mailer\Transport\AbstractTransportFactory` which will simplify the new factory:
+
+.. code-block:: php
+    
+    final class CustomTransportFactory extends AbstractTransportFactory
+    {
+        public function create(Dsn $dsn): TransportInterface
+        {
+            // create and return the transport
+        }
+
+        protected function getSupportedSchemes(): array
+        {
+            return ['custom_schema'];
+        }
+    }
+
+Finally, declare the new factory in setting tag the tag `mailer.transport_factory`:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/services.yaml
+        mailer.transport_factory.custom:
+            class: App\CustomTransportFactory
+            parent: mailer.transport_factory.abstract
+            tags:
+              - {name: mailer.transport_factory}
+
+    .. code-block:: xml
+
+        <!-- config/services.xml -->
+        <service id="mailer.transport_factory.custom" class="App\CustomTransportFactory" parent="mailer.transport_factory.abstract">
+            <tag name="mailer.transport_factory" />
+        </service>
+
+    .. code-block:: php
+
+        // config/services.php
+        $services->set('mailer.transport_factory.custom', App\CustomTransportFactory::class)
+                 ->parent('mailer.transport_factory.abstract')
+                 ->tag('mailer.transport_factory');
+
 Creating & Sending Messages
 ---------------------------