minor symfony#13786 Update mailer.rst (abdounikarim)

This PR was merged into the 4.4 branch.

Discussion
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Update mailer.rst

Hello,

I just add an alternative to use a configuration in test for Mailer.

Commits
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60a26bb Update mailer.rst

Author: javiereguiluz

mailer.rst

@@ -994,6 +994,14 @@ environment:
             ],
         ]);
 
+You can also disable delivering with an environment variable in your `.env.test` file
+(or you can create a `.env.test.local` file for example) :
+
+.. code-block:: bash
+
+    # .env.test
+    MAILER_DSN=null://null
+
 .. note::
 
     If you're using Messenger and routing to a transport, the message will *still*