Gates Backs E-Mail Stamp in War on Spam:
“Every proposed scheme will break parts of the way e-mail works today,” said Hans Peter Brondmo, a senior vice president of Digital Impact who has represented big e-mailers in the spam technology negotiations. The challenge, he said, is to find a system that will require as little retrofitting as possible to e-mail systems.
I think any system that requires you to pay to send or receive email is a bad idea or at least breaks part of what makes email work so well.
Re-reading this article, Tim also proposes a “stamp” of sorts, though of smaller denomination, but I think his ideas about whitelisting authorized relayers, instead of individual senders, is the best part of it. Right now, whitelists are at the individual level: you have to look at and approve everyone who wants to send you mail. It would be far simpler to add authoritative relayers (USPS.gov, etc. as Tim suggests) and let them handle the security and authentication.
Add to this the notion of signed email, either PGP/gpg or S/MIME, and the chances of getting something you didn’t want should go way down.