Another gateway box question

[windependence]

Mods please move this if it's not in the right place.

I have a pfsense box set up in a client's office. The internal LAN is split into basically two networks, one has the 100.0.0.0/24 scheme and the other is 192.168.1.0/24. The pfsense box has an internal address of 100.0.0.111 which is what the old gateway was on the hardware router. The external address is one of several they have available from their ISP.

Their internal Windows workstations are all on the 100.0.0.0/24 network, but I have one box, their mail server, which is 192.168.1.20. Here is the problem. I can't see the mailserver on the network when I am logged in on the 100.0.0.0/24 network. I thought since the pfsense box is a router, it would route traffic between the two networks. Networking has never been my real strong point but I do understand the basics. The Windows admin says he didn't want the mailserver on the same network in case there was a virus outbreak. Is there another way to do it without using a different network? Any ideas would be greatly appreciated.

-Tim