The October issue of the TechNet Magazine has an interesting article covering the Custom Weigth Lists (CWLs), which were introduced back in Exchange Server 2003 SP2. The article is written by Cam Frenette (Software Design Engineer in Test at Microsoft who focuses mainly on anti-spam technologies for the Technology Care & Safety team) and Alexander Nikolayev (Program Manager at Microsoft in charge of server-side protocols, Transport Core, and anti-spam components for Exchange and Windows Servers).
Read it here.