There are some good potential economic mechanisms using micro-payments. If it costs even a nickel to send an email that would greatly reduce spam. Money is not even necessary - suppose that when an email was sent a response was sent back saying in order to accept this email you must factor this number and send me back the factors - if the factorization took even a few micro-seconds that would not be an issue for non-spammers but would shut spammers down.
Alex Tabarrok
