hmmm relays and header forging........ it's actually stated in the same set of RFC's pertainting to SMTP transfer that in order for an smtp messaging system to meet standards it MUST have the capability of keeping the original headers intact. Without getting into too much detail about spiffy proxy chaining, relay hopping, and masquerading theres other means for the good mail server admin to change these headers. The best way to falsify such headers would be via a set of address rewrite rules using M4 and sendmail. To my knowledge there arent too many other smtp servers out there that have that level of control/editing so easily accessed....

course... if anyones ever looked at sendmails M4 macro's... easily accessed is a relative term.... The macro's are quite powerful but cryptic in the least... and many of the techniques used in advanced troubleshooting are the same techniques that can be used for spiffy relay hop / header forge tricks....

Of course, to the good mail admin, M4 is a godsend in it's versatility of performing nearly any task one could dream of on an smtp envelope