Fixed sending MDN messages. According RFC2298:
The From field of the message header of the MDN MUST contain the
address of the person for whom the message disposition notification
is being issued.
The envelope sender address (i.e., SMTP MAIL FROM) of the MDN MUST be
null (<>), specifying that no Delivery Status Notification messages
or other messages indicating successful or unsuccessful delivery are
to be sent in response to an MDN.
git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@7035
7612ce4b-ef26-0410-bec9-
ea0150e637f0