Need to route email traffic through multiple ethernet ports

Hey guys,

Please bear with me, and feel free to ask for clarification if I'm not specific enough in certain spots.  I was hired as a Windows Server guy, and got thrown into installing, configuring and maintaining a Mac server we're going to be switching our company email over to. :(

Below I've crudely drawn a map of how our network needs to be configured:
        ___________
        |PIX Firewall|
        -------------
         |     _____|____
         |    |Spam Filter|
         |     -----------
       _|________|____
      |Mac Email Server|
       -----------------

All internal email (company-to-company) needs to bypass our spam filter, so it should be going straight from the email client on a user's PC, through our PIX Firewall (depending on whether they're on or off our corporate network), to our Mac OS X (10.5.7) Email server running Squirrelmail 1.4.17 on ports 25, 80 and 110.

However, all mail going to, or coming from external domain servers need to be routed through our spam filter (SonicWall ES500) on port 25, and then to our Mac Email server, or vice-versa for outgoing mail.

The Mac server *does* have 2 ethernet ports, just need to know how to tell it to route traffic appropriately.

Thank you for your help in advance, and again, my apologies if there's any vital system information I might've left out!