Merge branch '5.3' into 5.4

* 5.3:
  Added cautionbox for sender

Author: javiereguiluz

reference/configuration/framework.rst

@@ -3375,8 +3375,9 @@ sender
 
 **type**: ``string``
 
-Sender used by the ``Mailer``. Keep in mind that this setting override a
-sender set in the code.
+The "envelope sender" which is used as the value of ``MAIL FROM`` during the
+the `establishment of the SMTP session`_. This value overrides any other sender
+set in the code.
 
 recipients
 """"""""""
@@ -3641,3 +3642,4 @@ to know their differences.
 .. _`Microsoft NTLM authentication protocol`: https://docs.microsoft.com/en-us/windows/win32/secauthn/microsoft-ntlm
 .. _`utf-8 modifier`: https://www.php.net/reference.pcre.pattern.modifiers
 .. _`Link HTTP header`: https://tools.ietf.org/html/rfc5988
+_ ..`establishment of the SMTP session`: https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol#SMTP_transport_example