Accessing other network services through VPN?

I need to be able to access my internal mail server through my VPN connection but am unable to.

Here's some background...

My VPN server is a G4 Xserve that I'm also using primarily as my OD Master. It is running v10.4.7. I have this server connected to an internal 3Com Gigabit switch using both NICs with an LACP connection. This server is also doing internal DNS. I do not have the Firewall services turned on on this server as it is internal and deemed an acceptable risk since it sits on the inside interface of my Cisco PIX firewall. I have setup the VPN services on it and am able to connect properly and access my local AFP services on another local Xserve. I can also access the internet and load external websites. What I need to be able to do for my users is allow them access to our internal mail server. This server can normally be accessed internally using either the DNS name or an IP address, and communicates over port TCP port 510.

The mail server in question is a FirstClass mail server hosted on another Xserve running v10.4.7. This mail server is sitting on the DMZ of my PIX firewall (IPs: 192.168.100.1 - 192.168.100.10). The PIX is doing DHCP and NAT for my internal network (IPs: 192.168.10.1 - 192.168.10.256)

So far I've tried (without success):

- switching the VPN configuration from Private to Public
- turned off "Send all traffic through VPN" in Internret Connect
- turned on NAT services (IP Forwarding Only) on the VPN server

I'm not an expert on Mac OS X Server and I was hoping that someone out there might have an answer for me on how I can get to my mail server on my DMZ.

2.66 Mac Pro/2.16 17 MacBook Pro/MP2100 Mac OS X (10.4.7)

G5Dual2.0