Outlook cannot connect to Exchange Server after upgrade to SP1

I have a notebook running Windows 2003 Server. I also have Microsoft Office
2003 on the notebook. When connected to my network, Outlook sees my Exchange
2000 server without a problem.

I also have a CheckPoint firewall. The firewall provides for my VPN access
while out of my network. I can hook up to the Internet, start up my VPN
connection and Outlook behaves as though I was on net. Now, I have to make
some adjustments to the hosts file in c:\windows\system32\drivers\etc, but
once I have the right mapping, (mailserver --> IP), Outlook is fine.

Since I installed SP1 for Windows 2003, Outlook is unable to locate my
Exchange server. I can use OWA. But I can't use Outlook. My DNS is pointing
to my ISP, but that has not changed since before SP1. I was able to add an
entry to the hosts file and Outlook was fine.

Is there anything I can do about this or am I stuck to OWA when I am outside
the network?

Thanks.

M.