Unles you have a block of legal ip addresses you can use for your workstations you have to use nat.
It's not that hard to do, instal routing and remote acces (I like to do this manually without the wizzard) and install the nat protocol.
Bind it tothe outgoing nic, and maybe add a port 25 forwarding to your mail server, if this has to be accessible from the outside (if you use etrn or bsmtp).
Let yopuur mail provider forward the ail to your legal ip address (the nic on the outside) and of you go.
If w2000 is already installed. it should not take you more than ten minutes.
Good Luck
Ron.