As an interim upgrade path ISPs adopting it could be to bounce messages with out payments, and include a nonce, and instructions to resend including the nonce. Set up the filter so that the second post gets through. Spammers often don't have forged reply addresses for obvious reasons.
I don`t think we even want to get into the issue of ISPs being involved in hashcash or any other form of postage, a better system is a simple user<->user model where the users mail reader can be configured to filter email without hashcash, procmail scripts to do this wouldn`t be hard either I imagine (disclaimer: I`m no procmail expert, and this may be entirely wrong). Any form of ISP censorship, bouncing or filtering is a bad move. Datacomms Technologies data security Paul Bradley, Paul@fatmans.demon.co.uk Paul@crypto.uk.eu.org, Paul@cryptography.uk.eu.org Http://www.cryptography.home.ml.org/ Email for PGP public key, ID: FC76DA85 "Don`t forget to mount a scratch monkey"