feature #18778 [Mailer] [Smtp] Add DSN param peer_fingerprint for fingerprint verification (xdavidwu)

This PR was merged into the 6.4 branch.

Discussion
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[Mailer] [Smtp] Add DSN param `peer_fingerprint` for fingerprint verification

Refs
* symfony/symfony#51450

Commits
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dbfcbb1 [Mailer][Smtp] Add DSN param `peer_fingerprint` for fingerprint verification

Author: OskarStark

mailer.rst

@@ -334,6 +334,17 @@ the application or when using a self-signed certificate::
 
     $dsn = 'smtp://user:[email protected]?verify_peer=0';
 
+TLS Peer Fingerprint Verification
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Additional fingerprint verification can be enforced with the ``peer_fingerprint``
+option. This is especially useful when a self-signed certificate is used and
+disabling ``verify_peer`` is needed, but security is still desired. Fingerprint
+may be specified in sha1 (40 characters) or md5 (32 characters)::
+
+    $dsn = 'smtp://user:[email protected]?peer_fingerprint=6A1CF3B08D175A284C30BC10DE19162307C7286E';
+
+
 Overriding default SMTP authenticators
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~