[Mailer] add tag and metadata docs

mailer.rst

@@ -730,6 +730,47 @@ adding a text header ``X-Transport`` to an email::
     $email->getHeaders()->addTextHeader('X-Transport', 'important');
     $mailer->send($email);
 
+Adding Tags and Metadata to Emails
+----------------------------------
+
+Certain 3rd party transports support email *tags* and *metadata*. These can be
+used by the 3rd party service for grouping, tracking and workflows. The
+:class:`Symfony\\Component\\Mailer\\Header\\TagHeader` and
+:class:`Symfony\\Component\\Mailer\\Header\\MetadataHeader` headers can be
+added to your email. If your transport supports these headers, they will be
+applied in the format specific to the transport.
+
+.. versionadded:: 5.1
+
+    The :class:`Symfony\\Component\\Mailer\\Header\\TagHeader` and
+    :class:`Symfony\\Component\\Mailer\\Header\\MetadataHeader` classes were
+    introduced in Symfony 5.1.
+
+The following 3rd party transports currently support tags and metadata:
+
+* Postmark
+* Mailgun
+* MailChimp
+
+For example, say you want to tag an email and add some metadata::
+
+    use Symfony\Component\Mailer\Header\MetadataHeader;
+    use Symfony\Component\Mailer\Header\TagHeader;
+
+    $email->getHeaders()->add(new TagHeader('password-reset'));
+    $email->getHeaders()->add(new MetadataHeader('Color', 'blue'));
+    $email->getHeaders()->add(new MetadataHeader('Client-ID', '12345'));
+
+When sending this email with a transport that supports tags and metadata, the transport
+will convert these to their appropriate format. If using a transport that does not
+support tags and metadata, they will be added as custom headers:
+
+.. code-block:: text
+
+    X-Tag: password-reset
+    X-Metadata-Color: blue
+    X-Metadata-Client-ID: 12345
+
 Development & Debugging
 -----------------------