While not a solution for email, Phil Haack talks about a simple way to help prevent spam bots from attacking your forms on your web sites. By using an invisible captcha, one can defeat most bots.
First, what is a Captcha? It is a challenge - response test to help determine that you are actually a human. You've undoubted already came across a few of these during a sign up process of some text with a grid or a line through it asking what it says.