Hashcash is an interesting method that can potentially stop spam. Hashcash adds X-Hashcash: headers to email messages. The generation of the headers will take some CPU cycles, so it is not feasible for spammers to put this header into each of their email messages. On the receiving end, it is a very inexpensive algorithm to validate the header.
This has been built into SpamAssassin 3.0, so if people start using these headers SpamAssassin will give these messages a lower spam score. Here are the instructions for SpamAssassin
This is related to the other ways to stop spam. Habeas has a legal solution, while Bonder Sender has a financial solution
I hope that someone will create a Apple Mail plugin that will create these hashes. I would be glad to install it.
Also I am not sure that this is really going to stop spam. In order to stop spam, all mail servers and mail clients would have to add these headers.