VPN and Outlook

Hi All,

I have been having a problem for a long time connecting to Outlook when my customers are using a VPN connection. My users are connecting to a Microsoft VPN 2003 Server(it had the same problem using W2K Server). After they are connected using the VPN connection they need to connect to our Exchange 2000 (SP3) Server using Outlook 2000/XP. The connection will time out after a few minutes without making the connection. I have tried to change the MTU(below 1500) size and sometimes they can connect and other times they can not. Any ideas on what I can check? My boss is the main user and he has the most problems.

Thanks,
Bill