A great Open Source project for gaining understanding about e-mail systems, including an in-depth look at SMTP and POP3, is the Java-based Apache JAMES Project. Although JAMES has the unfortunate shortcoming of being built around the now defunct and unsupported Apache Avalon Framework, it’s still a fantastic learning tool for understanding email protocols, mail delivery, and spam filtering. Not only that, it’s a fully functional, enterprise-ready mail server that can be up and running with minimal configuration.
One technology implemented by JAMES for spam filtering is real-time DNS blacklists. DNSBLs identify the IP addresses of potential spam sources and machines known to be delivering spam (as determined by the sometimes controversial policies of the list owner). Spam blacklists date back to 1996 with Paul Vixie’s Mail Abuse Prevention System, and are now used by ISPs and corporate mail systems around the world. Countless organizations maintain blacklists, and Web sites like MX Toolbox permit ad hoc queries of IP addresses against dozens of published lists.