Recently I installed Exchange 2003 SP2 on an SBS server that did NOT have SBS SP1 on it. After installing the Exchange service pack, remote users could not logon just using their username. They had to include the domain name as well — format: domain\username. I tried running the KB 832539 fix that came out after SBS SP1, but it would not run, saying it could not find Exchange 2003 SP1.
If you have a similar situation, here is the workaround that will allow a user to login to OWA by just using their username:
1. Start up IIS
2. Under the Default Web Site locate the Exchange virtual directory (the one labeled Exchange, not the ExchWeb one)
3. Right click on the Exchange virtual directory and click on Properties
4. Go to the Directory Security tab
5. Click EDIT in the Authentication and Access Control section
6. The default domain name field will currently contain a single slash: \
7. Replace the single \ with the Netbios Domain name* of your SBS server and OK the changes
8. Then stop and restart the Default Web site
* If you are not sure of the Netbios domain name of your server, go to AD Users & Computers, then right click on your server name, and click Properties. Your Netbios domain name is listed in the Pre-Windows 2000 domain name field.