8000 Incorporated (EIGH)

[P2O I'm from MO]

The IP 'trail' is in the message header.

You can forge any SMTP header before sending it, even before passing it to a legit mailserver. However, every SMTP server that is between the sender and the receiver should add its own Received: header. At least, that's open standard and on the other hand, not everyone sticks to the standards. That's why the only thing you can rely on is your own mail exchange server, which has logged the IP address of the last server that passed the mail.

That being said...I've still to find any concrete evidence of anyone being able to spoof the TRUE ip of the machine that sends out an e-mail.