At the company I work and also on the server this blog is hosted we use SpamAssassin to filter our (clients) e-mail messages. In our SpamAssassin configuration we use a few specific DNS based email blacklists. (Commonly called Realtime blacklist, DNSBL or RBL). If the IP address from which the e-mail is send has been blacklisted, some email you send may not be delivered. Email blacklists are a common way of reducing spam.
To check ip adresses against our configered DNSBL's I crafted up a webbased application that reports if a submitted IP is listed or not listed in one of our used DNSBL's.