Real-time Blacklists (RBLs) are commonly used to block mail from known spammers or servers that can be used to send spam (open relays and the like). I found a lot of good info on setting up rbldnsd here. I use Bind 9 on my DNS servers so that I can run rbldnsd on the same IP as bind. Note: Setting up and configuring Bind 9 for basic DNS is beyond the scope of this document.