minor #12422 [Mailer] Add support for multiple mailers (hiddewie)

This PR was squashed before being merged into the 4.4 branch (closes #12422).

Discussion
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[Mailer] Add support for multiple mailers

#12259 and symfony/symfony#33409

~~Do we need a `versionadded:: 4.4`?~~

![image](https://user-images.githubusercontent.com/1073881/66234058-05c5d680-e6ed-11e9-963f-d2a25c508aee.png)

Commits
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aeedbd2 [Mailer] Add support for multiple mailers

Author: javiereguiluz

mailer.rst

@@ -697,6 +697,38 @@ you have a transport called ``async``, you can route the message there:
 Thanks to this, instead of being delivered immediately, messages will be sent to
 the transport to be handled later (see :ref:`messenger-worker`).
 
+Mutliple email transports
+-------------------------
+
+.. versionadded:: 4.4
+
+    The option to define multiple email transports was introduced in Symfony 4.4.
+
+You may want to use more than one mailer transport for delivery of your messages.
+This can be configured by replacing the ``dsn`` configuration entry with a
+``transports`` entry, like:
+
+.. code-block:: yaml
+
+    # config/packages/mailer.yaml
+    framework:
+        mailer:
+            transports:
+                main: '%env(MAILER_DSN)%'
+                important: '%env(MAILER_DSN_IMPORTANT)%'
+
+By default the first transport is used. The other tranports can be used by adding
+a text header ``X-Transport`` to an email:
+
+.. code-block:: php
+
+    // Send using first "main" transport ...
+    $mailer->send($email);
+
+    // ... or use the "important" one
+    $email->getHeaders()->addTextHeader('X-Transport', 'important');
+    $mailer->send($email);
+
 Development & Debugging
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