Connect using Sonicwall VPN client problem

I am having trouble connecting to the Exchange Server behind Sonicwall
Firewall using Outlook on client Machine.

Configuration:
Server-Exchange 5.5 in DMZ behind Sonicwall Firewall.
Client-Home computer on local LAN, through a BSD gateway box that dials (56K
modem) local ISP. (Other VPN connections work fine - i.e. Citrix)

Symptoms:
*On "check names" prompt in Services, server and mailbox are found sometimes
but not others. When not found it says "The name could not be resolved."
When the mailbox and server are found, it never really connects to the
mailbox...getting a message like 'folder not available'.

I think this is an authentication issue, probably due to some port number
not being open on the firewall.

Am I warm? Does anyone have any thoughts?

Rob Fisch