Outlook 2003 Not responding when connected to VPN

Hi

I am Remote access admin for a company and we have users who are connected remotely using the Nortel software V04_65.320. There are sometimes when we have users who are not able to access the outlook client at all when connected. They get a message ” Exchange Server is unavilable”

We tried to reduce the MTU scrit for the nortel adapter key.. but it did not work…

Any ideas?

Thanks
Deepak