  Proof-of-work (POW) schemes have been proposed in the past. One prominent
system is HASHCASH (Back, 2002) which uses cryptographic puzzles . However,
work by Laurie and Clayton (2004) has shown that for a uniform proof-of-work
scheme on email to have an impact on SPAM, it would also be onerous enough to
impact on senders of "legitimate" email. I suggest that a non-uniform
proof-of-work scheme on email may be a solution to this problem, and describe a
framework that has the potential to limit SPAM, without unduly penalising
legitimate senders, and is constructed using only current SPAM filter
technology, and a small change to the SMTP (Simple Mail Transfer Protocol).
Specifically, I argue that it is possible to make sending SPAM 1,000 times more
expensive than sending "legitimate" email (so called HAM). Also, unlike the
system proposed by Debin Liu and Jean Camp (2006), it does not require the
complications of maintaining a reputation system.

Networking and Internet Architecture;Cryptography and Security