The value of creating a “whitelist” for email, as opposed to a blacklist. (In only 35 steps!)
The basic idea is that you create a system whereby, in order for someone to send you an email, you require them to send a confirmation that their listed email address is actually a valid one. (Many spam-emails are automatically generated and don’t have a valid reply-to address.)
(I found this via Scott Rosenberg’s Salon Blog. I don’t normally read Lockergnome, though maybe I should start.)