Hashcash is a method whereby you make the other end perform some computation in order to send you a message.

The computation isn’t too taxing for a few messages; but a lot of messages will require a fair amount of power in order to send them. Thus, you increase the “cost” of sending you messages. You can even vary the amount of processing necessary by incoming address; unknown addresses cost more, trusted addresses cost less (or nothing).

camram is a system (and a website) that contains the code and instructions for setting up your MTA for use with hashcash.