I recently became aware of this problem; with all the Twitterer's of the world wildly posting their e-mail everywhere all willy-nilly, their email addresses are subject to the harvesting by robots. Robots crawl websites like twitter (and yes they even crawl my site) so sooner or later you'll wind up on a spammer list. So how to protect yourself? I use...

MailServer is my favorite SMTP POP3 mail server application because it's free, simple, complete, and has everything you could ask for in a mail server, without the fluff. It has SQL support for large E-mail servers with many users and E-mails, or it can use an internal Database for the simpler tasks.