This is a breif outline of our Custom Filtering:
Everyone should know that Mail Filtering is constantly evolving. So ones systems must also evolve in order to stay ahead of the spammers. Since Spam Assassin and ClamD were written to run on Linux they were designed as Multithreaded applications. This is why our filtering servers are running on 64 bit Centos.
Since SmarterMail has exposed using remote filtering servers for things like Spam Assassin, Bayesian Filtering, ClamD, and Razor we do this and balance each mail server between 3 filtering servers to be able to absorb any level of attack.
We also use ClamD from a remote since this dramatically reduces the load at the mail server for looking through emails for viruses.
The Last action is when it is all brought back to the mail server and the point where weights are imposed against a specific email.
All RBL lists have a weight associated. Also you can see we assign a weight to the email for things like Spam Assassin and Bayesian (Or "Words" These are collected from emails) At a weight of 35 the email will be automatically deleted.