The stamps on a received mail should get semi-transparant post marks for each hop from the respective server. Could be generated from the mail header info.
But serious question: where would I "inject" my stamp generation on the mail process? Would need to be a client plugin or something like that, I guess?
Just remembered: this was one of my early childhood games (My little Post office - "with the world-stamp collection on the Internet"). The TIVOLA intro was dope!