minor #19455 [Mailer] Add warning about custom SES headers (KDederichs)

This PR was merged into the 5.4 branch.

Discussion
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[Mailer] Add warning about custom SES headers

Fixes symfony/symfony#45168 by mentioning that the provider needs to be ses+https in order to transmit custom headers.

Commits
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cb44ce9 Add warning about custom SES headers

Author: javiereguiluz

mailer.rst

@@ -201,6 +201,11 @@ Provider              SMTP                                                     H
 
         The ``ping_threshold`` option for ``ses-smtp`` was introduced in Symfony 5.4.
 
+.. caution::
+
+    If you need to send custom headers to receive them later via webhook using the `Amazon SES` transport
+    be sure to use the ``ses+https`` provider since custom headers are not transmited otherwise. 
+
 .. note::
 
     When using SMTP, the default timeout for sending a message before throwing an