I was pondering the same question awhile ago. After poking around in the system and reading the temporary scratch files that the system created by the mailer, I noticed that the mail was being preceeded by a seperate header packet which was not included in the message. Sending mail to myself caused the system to create two temporary files in the process. One of them was called "SF" and the other was "QF". I don't know what the letters stand for, and this is probably just how CMU does it, other sites may be different. Anyway, in my test mail to myself, the SF file contained: #From |<mg5n+@andrew.cmu.edu>| #To |mg5n+@andrew.cmu.edu| #Auth |26634;andrew.cmu.edu;Matthew J Ghio| and the QF file contained the actual text of the message plus the headers that you see. So the email is actually sent as two seperate packets of data, the headers you see are just there for looks, the actual delivery info is hidden behind-the-scenes. Does anyone else have any description of "standard" methods of handling internet e-mail?