> The calculation takes only a second or so per piece of email, so it is an
> insignificant cost for most users. But spammers who send out thousands of
> emails must now customize each one with separate headers and a separate
> computation. The total computational costs for them are much higher and
> it automatically puts a limit on how much spam they can afford to send.
The problem should be NP. Thus my mail software only takes a millisecond to verify the message, but the spammer still takes a second to send it. (Otherwise you've increased their sending costs, but also my annoyance on the other end.)
-- firstname.lastname@example.org Eliezer S. Yudkowsky http://pobox.com/~sentience/AI_design.temp.html http://pobox.com/~sentience/sing_analysis.html Disclaimer: Unless otherwise specified, I'm not telling you everything I think I know.