Exchange dialup and network connectivity

I have moved exchange 5.5 from an NT 4.0 machine to a Windows 2000 Domain controller. The Exchange account uses a dialup connection to send and recieve Email.There are also shared drives on this Domain controller. There are four sites located in four different cities all connected by fractional T1’s and each city has its own Domain controller. Now that the dialup is working when it dials out it causes the remote sites to lose connectivity to the Domain controller with Exchange on it. When the dialup is finished connectivity to the Domain controller returns. The issue is only with the remote sites not the site that the Domain controller resides on. I would appreciate any thoughts anyone has.