If someone tries to send mail and their key is not on the
recipient's list, the mail is returned to them until they can
perform a Hashcash calculation consuming a non-trivial amount of CPU
time, at which point their key is placed on the recipient's list,
and the sender can retry to send the message.
Hashcash has one big problem: worms. CPU time is free for them..
There are many reports in the press suggesting that spammers are using
worms such as "Jeem" to deploy open relays; they could also use them
to deploy hashcash generators.
- Bill