smtp server in the middle gateway

I was wondering if anybody knew how to set up a smtp server in the middle gateway that that can monitor and deny email based on various things for the entire network.

I already have a linux router set up in which all the traffic routes through. This makes it real easy to monitor attacks and firewall ip addresses that may be attacking in some way. It protects the entire network through one machine.

I know sniffing software can monitor the email as it goes by. I want to know how I can get that router to deny email based on perhaps RBL or some header in the email like maybe the from address or subject line.

Thanks