Yes, this is behind a router. Since this is my "lab" I actually only have 2 boxes behind it, an ISP Config machine (the one that's giving me mail problems) and an Ubuntu desktop, nothing else. The router/firewall I'm using is Untangle. For testing, I turned off the firewall portion and am only using the port forwards. I was still having problems, but
! - more troubleshooting, but progress!
I went into Untangle and turned off EVERYTHING except port-forwarding and I can now receive mail. I will continue to test until I can determine which module is causing the problems.