Cost-based Anti-spam Systems - Proof-of-work Systems

Proof-of-work Systems

Proof-of-work systems such as hashcash and Penny Black require that a sender pay a computational cost by performing a calculation that the receiver can later verify. Verification must be much faster than performing the calculation, so that the computation slows down a sender but does not significantly impact a receiver. The point is to slow down machines that send most of spam—often millions and millions of them. While every user that wants to send email to a moderate number of recipients suffers just a few seconds' delay, sending millions of emails would take an unaffordable amount of time. This approach suffers when sender maintains a computation farm of their own or from zombies.