Hashcash schemes are anattempt to slow down the rate that spammers can send email by makingthem burn CPU time on some non-trivial calculation. This soundssuperficially attractive (and has ever since 1997), but will neverbe adopted; in fact, the idea is dead on arrival.

To see why, we only need to observe that hashcash has a disproportionateeffect on the heaviest senders of email; in fact, this is exactly whatit is designed to do. Now think of what this does