Merge branch '5.1' into 5.2

* 5.1:
  Complete documentation about mailer integration

Author: wouterj

mailer.rst

@@ -46,6 +46,7 @@ over SMTP by configuring the DSN in your ``.env`` file (the ``user``,
             xsi:schemaLocation="http://symfony.com/schema/dic/services
                 https://symfony.com/schema/dic/services/services-1.0.xsd
                 http://symfony.com/schema/dic/symfony https://symfony.com/schema/dic/symfony/symfony-1.0.xsd">
+
             <framework:config>
                 <framework:mailer dsn="%env(MAILER_DSN)%"/>
             </framework:config>
@@ -55,6 +56,7 @@ over SMTP by configuring the DSN in your ``.env`` file (the ``user``,
 
         // config/packages/mailer.php
         use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
+
         return static function (ContainerConfigurator $containerConfigurator): void {
             $containerConfigurator->extension('framework', [
                 'mailer' => [
@@ -83,7 +85,7 @@ sendmail      ``sendmail://default``                    Mailer uses the local se
 Using a 3rd Party Transport
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Instead of using your own SMTP server, you can send emails via a 3rd party
+Instead of using your own SMTP server or sendmail binary, you can send emails via a 3rd party
 provider. Mailer supports several - install whichever you want:
 
 ==================  ==============================================

reference/configuration/framework.rst

@@ -178,6 +178,7 @@ Configuration
 
   * :ref:`dsn <mailer-dsn>`
   * `transports`_
+  * `message_bus`_
   * `envelope`_
 
     * `sender`_
@@ -926,6 +927,8 @@ This setting is automatically set to true when one of the child settings is conf
 
 .. _http-headers:
 
+.. _http-headers:
+
 headers
 .......
 
@@ -3007,6 +3010,18 @@ transports
 A :ref:`list of DSN <multiple-email-transports>` that can be used by the
 mailer. A transport name is the key and the dsn is the value.
 
+message_bus
+...........
+
+.. versionadded:: 5.1
+
+    The ``message_bus`` option was introduced in Symfony 5.1.
+
+**type**: ``string`` **default**: ``null`` or default bus if Messenger component is installed
+
+Service identifier of the message bus to use when using the
+:doc:`Messenger component </messenger>` (e.g. ``messenger.default_bus``).
+
 envelope
 ........
 
@@ -3061,6 +3076,7 @@ recipients set in the code.
 
         // config/packages/mailer.php
         namespace Symfony\Component\DependencyInjection\Loader\Configurator;
+
         return static function (ContainerConfigurator $containerConfigurator): void {
             $containerConfigurator->extension('framework', [
                 'mailer' => [