Microsoft had about sixteen knowledgebase articles regarding setting up static port mapping to connect from Outlook to Exchange using MAPI/RPC. Now there is one with all the pertinenet information including Exchange 5.5, 2000 and 2003.
In a side bar, this is NOT a recommended configuration for connecting to Exchange from the Internet. You should use VPN or the new Outlook 2003 RPC over HTTP capabilities. However, you might consider this if there is no other alternative, or as a way to connect through internal Firewalls separating workstations and servers.