OK - I mis-wrote on the NTFS folder permissions - Webdata and Webtemp are Exchange 5.5 folders, not 2K. For 2K, you need to check the minimum level of permissions on the following: \exchsrvr\exchweb and \exchsrvr\Mailroot folders - Authenticated Users (or Everyone) full control; c:\Inetpub and c:\Inetpub\wwwroot folders - Authenticated Users (or Everyone) Read & Execute, Read, and List Folders Contents.
Also, one thing I forgot to mention, in the Basic Authentication settings, you should see a box for the domain name. Be sure that your correct NetBIOS domain name is shown here. If there is a "\" or if the box if blank, change it to your domain name.
Once that's done, try to run Outlook Web Access internally by going to http://[servername]/Exchange. See if this works. If it doesn't, try just going to http://[servername] and see if you can at least get to the web site itself. If you CAN get to both Exchange and the web site internally, then try externally again see what happens. Good Luck! Let me know the results of this.