Potential fake E-Mail or Spam Mail Relay The envelope sender may not be the same as the "From:" line in the message itself; it can be used by a mail server to prevent the message from being sent. And, of course, there's nothing to prevent the sender from using an address that doesn't exist, or that the sender doesn't own. Source: http://www.honet.com/Nadine/Envelope-Senders.htm