We use MXLogic Spam Control and it is awesome, has outperformed the last 3 different Spam apps we have purchased, I’m not sure they will let you brand their service, actually not sure anyone will for Spam defenses like you suggest but it’s worth a try, contact MXlogic and ask.
Why not do it in the cloud and become a Google partner? Their Postini services are very effective and the traffic does not have to reach the client’s network so they save on network bandwidth at the edge. You get so much more than anti-spam and anti-virus with this solution. It is well worth the look I believe.