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