X-Hashcash: 0:030626:adam@cypherspace.org:6470e06d773e05a8
You can choose how much work you want to do and the recepient can specify thresholds for minimum work required. This header is all they have to store, and it's easily stored through existing email infrastructure. Your mail server can do it without your client's help, too. If you spent a second on a proof I'm sure grandma wouldn't notice, but it would be very difficult for spammers to do the same.
For one, it'll only work if both sender and receiver use it. Which means for 99.99% of mail traffic right now, it is utterly meaningless.
edit: Plus you also literally wasted everyone's time and energy.
I'd rather favor some kind of mechanism like grey-lists that don't require sender opt-in otherwise it'll be a dead technology just like GPG. (You can probably count the number of GPG emails within 1000 average mails on one hand)
