[Mailer] [Smtp] Document auto_tls param

mailer.rst

@@ -347,6 +347,30 @@ may be specified as SHA1 or MD5 hash::
 
     $dsn = 'smtp://user:pass@smtp.example.com?peer_fingerprint=6A1CF3B08D175A284C30BC10DE19162307C7286E';
 
+Disabling automatic TLS
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 7.1
+
+    Disabling automatic TLS was introduced in Symfony 7.1.
+
+By default, mailer will check if OpenSSL extension is enabled and if SMTP server
+is capable of STARTTLS, it will issue this command to enable encryption on stream.
+This behavior can be turned off by calling ``setAutoTls(false)`` on ``EsmtpTransport``
+instance, or by setting ``auto_tls`` option in DSN::
+
+    $dsn = 'smtp://user:pass@10.0.0.25?auto_tls=false';
+
+.. caution::
+
+    It's not recommended to disable TLS while connecting to SMTP server over
+    internet, but it can be useful when both application and SMTP server are in
+    secured network, where there is no need for additional encryption.
+
+.. note::
+
+    This setting works only when ``smtp://`` protocol is used.
+
 Overriding default SMTP authenticators
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~