Access MS Exchange through VPN

At the office I’m using a Linksys WRV54G router and connecting to it with Linksys VPN Client on the remote client computer. I can get a VPN connection with no problem, but I cannot get the remote client to connect to the Exchanage server. One clue might be that I have to identify the host server by it’s IP address, not it’s host name. Any ideas what is going on here?