We should stick stamps on emails just because it's cute.
Do we need more stamps when the attachments are larger? And/or is it more about the distance (hops) of travel?
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.
Can't wait for bouncing mails because there weren't enough stamps on it :D
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?

@bleeptrack How about instead of payment stamps adding a "received stamp". There's already a Received: header for every server that touched your e-mail (at least in theory).

You can always add a Comment: header, but it's meant to be human readable text - so there might be other comments in place already and your plugin has to identify the one with an embedded stamp.

Edit: having an online tool to decode your header if someone has a different client/webui would also be useful.