Exchange 5.5 & PPTP

I am having a problem with my RAS clients accessing their email. They get a message stating that the exchange server is unavailable. All the clients on the LAN have email and it is working fine internally. I believe it could have a relation with PPTP. Any ideas or suggestions?

We are running Exchange 5.5 (SP4) on a Winnt 4.0 (SP6) box.
Thanks in advance.