[lch]

You need a route between the networks so that they can find each other. If the server is behind a router, you could configure it to forward the game port to the external network (WAN, the internet), too. Or place it in the DMZ as a last resort if your router supports that and if you don't manage to do the port forwarding. If you can't get it to work this way then you need to use a VPN like Hamachi. This will install a virtual private network as network device and allow you to access the server over this VPN. PBEM involves mostly manual work of sending and receiving the emails with the turn files.