We should stick stamps on emails just because it's cute.
@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.