Background: We have a network with multiple child domains, each of which need to access OWA.
Problem: People don't want to put in Domain\Username to login. Typing is difficult and any keystrokes putting in the domain\ are calories just wasted. Our users want to just type in their username to login. This is easily done when you have one domain, and there are many posts that already cover this. What I had a hard time finding was how to do this with multiple domains in the forest. I noticed this was possible on our Exchange 2007 environment that was stood up well before I arrived, but I was upgrading to Exchange 2013 and couldn't figure out how to make this happen.
Solution: I noticed when I did a get-owavirtualdirectory command on the Exchange 2007 CAS server, it returned a value for DefaultDomain as \ and LogonFormat as "Username". Since I couldn't put in this value in the Exchange 2013 EAC, I tried it through PowerShell and it worked.
Set-OwaVirtualDirectory -identity "casservername\owa (default web site)" -LogonFormat UserName -DefaultDomain "\"
Update: I also found this setting can be done in IIS, and can also be changed for ActiveSync. To do this in IIS 7 or 8, go to the Default Web Site, then click the owa or Microsoft-Server-ActiveSync virtual directory. On the right, under IIS, double-click Authentication, then right-click Basic Authentication and choose Edit. For Default domain, input a \. Do not use quotation marks. Once you've made the change, do an IISRESET for changes to take effect.
