Is the server presently acting as a router between your LAN and the internet (WAN)? What happens (details, please; no 'doesn't work' descriptions) when your LAN client tries to access your server using the WAN interface URL's? Can you access the webmail service on your server by using LAN URLs?
In my vision, you want to set up the server as a router (I prefer HomeLanSecurity
to configure that) between the WAN and LAN sides. Then, configure the LAN clients to use the router as the gateway to the WAN side (method varies somewhat depending on the LAN client OS). That should do it. You can consolidate all of this into one tidy config file for all LAN hosts by using DHCP on the LAN.