Alternatively, just create electronic stamps* for email; cheap enough that it doesn't enter the minds of those that send a few emails per day but expensive enough to ruin the profit margins of spammers.

* I'm thinking something similar to SSL: write email, create hash (one hash per recipient), submit signing request for hash, CA signs request and returns stamp (certificate), stamp is transmitted along with email. Additionally, allow people to give each other (or sites/newsletters they would like to receive) 'rubber stamps', allowing them to create (free) stamps for emails addressed to each other... And since no idea of mine is ever original I'm certain several smarter people have already written multiple RFC's on this and the idea never took off because of something I'm overlooking at the moment. Ah well.