> 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