2. Navigate to Server Configuration > Client Access
3. Click on the CAS Server and go to the properties of owa(Default Website).
4. Click on Authentication tab of OWA website properties.
5. Modify the settings for Use form-based authentication: from Domain\user name to User name only option as shown below
6. Apply the changes. Exchange will show you a warning message to reset IIS, click on OK and exit the window
7. Open command prompt and run the command IISRESET /noforce(Use IISRESET if the /noforce fails).
Now open you Outlook Web App login page again, you can see it only asks for User name not the domain.
Very easy, implement it and relax. You can use the same settings for Exchange 2007 as well.