How can you block SPAM?

Derek Pomery asks: "My personal address, which I have kept relatively pure, is slowly being overwhelmed. Procmail doesn't seem to be the solution, since most of the e-mail addresses are forged. Does anyone have any ideas on the problem? The only thing I can think of is checking to see if the domain is valid. However, besides sending a verification e-mail from a throw-away address to check for a bounce (which would be an annoying kind of return spam), I can't think of a way to check for an account's validity (unless finger is allowed). Anyone have any spam killing strategies? Particularly for the annoying forged spam? Just some simple method to deep-six anonymous e-mails into /dev/null "