On my way to work this morning I was thinking of an article that I read on CNN (and other news sources) regarding charging a low fee on a per email basis, which would make it overly expensive for spammers to send out thousands-millions of emails a day. The reason I don't think that this is a good idea is a simple one. I subscribe to free mailing lists, such as Bugtraq. These mailing lists send me hundreds of emails a day, in turn they send out millions of emails a day to all of its subscribers; how good is a mailing list like that going to be if its users have to pay 30-40 a month? Most of its subscribers are hackers, in school... it would not be cost effective for them to remain on the list, Bugtraq would wither and die without them and there contributions.
Yahoo Groups is another fine example, its an Ad supported mailing list, I don't think Yahoo makes enough off the ads to pay for the amount of email they send, but if it becomes a paid service people will stop using it. E-Mail's strongest virtue is that it allows people to freely communicate (extra emphases on free) If you take that away, E-Mail will die.
I'd rather keep spam than try to fight it with this sort of tactic. There are all sorts of other topics that stem from email stamps, such as who gets the money for said stamps (Microsoft?) and how will it be enforced. Lets put this whole stamp idea, and Clippy for that matter, where it belongs - in the trash.