[Mailer] Mentioned the getMessageId() method

Author: javiereguiluz

mailer.rst

@@ -271,6 +271,17 @@ provides access to the original message (``getOriginalMessage()``) and to some
 debug information (``getDebug()``) such as the HTTP calls done by the HTTP
 transports, which is useful to debug errors.
 
+.. note::
+
+    Some mailer providers change the ``Message-Id`` when sending the email. The
+    ``getMessageId()`` method from ``SentMessage`` always returns the definitive
+    ID of the message (being the original random ID generated by Symfony or the
+    new ID generated by the mailer provider).
+
+    .. versionadded:: 4.4
+
+        The ``getMessageId()`` method was introduced in Symfony 4.4.
+
 The exceptions related to mailer transports (those which implement
 :class:`Symfony\\Component\\Mailer\\Exception\\TransportException`) also provide
 this debug information via the ``getDebug()`` method.