minor #14859 [Mailer] Update mailer.rst (cadjou)

This PR was submitted for the 5.2 branch but it was squashed and merged into the 4.4 branch instead.

Discussion
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[Mailer] Update mailer.rst

Add Tips for special characteres for user and pass

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Commits
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5d8a9e2 [Mailer] Update mailer.rst

Author: javiereguiluz

mailer.rst

@@ -65,6 +65,13 @@ over SMTP by configuring the DSN in your ``.env`` file (the ``user``,
             ]);
         };
 
+.. caution::
+
+    If the username, password or host contain any character considered special in a
+    URI (such as ``+``, ``@``, ``$``, ``#``, ``/``, ``:``, ``*``, ``!``), you must
+    encode them. See `RFC 3986`_ for the full list of reserved characters or use the
+    :phpfunction:`urlencode` function to encode them.
+
 .. caution::
 
     If you are migrating from Swiftmailer (and the Swiftmailer bundle), be