I have several clients that use VPN to connect to the 2003 SBS exchange server for email. Under VPN Properties-networking-tcp/ip-advanced, there is a checkbox. "Use Default Gateway or Remote Network" Somtimes it has to be checked and sometimes it has to be un-checked.
If a client calls me and tells me they cannot connect to the exchange server, I guide them through the process to either check it or uncheck it and it solves there problem. Is there any known reason for this, or just one of the mysteries of networking