Normally when you use OWA you see the initial logon page where the credentials are asked like Domain\User name:

When you want to use the UPN (in most cases identical to the e-mail address) you can set this on the OWA Virtual Directory in the Exchange Management Console:

When you select “Use forms-based authentication” and select “User principal name (UPN)” the initial login page changes accordingly:

When using TMG2010 in front of Exchange 2010 things are different. The logon form is now generated by TMG, and the Exchange server itself is set to basic authentication. By default the TMG logon page for Exchange is set to show the Domain\Username format and unfortunately there’s no easy way to change the logon page to show something different.

Please note that although the default page shows Domain\Username you still can use the UPN to logon!

To change the logon page to show a different text (or change the layout completely) you have to change the HTML pages. These pages can be found on the TMG server in directory C:\Program Files\Microsoft Forefront Threat Management Gateway\Templates\CookieAuthTemplates\Exchange\HTML. The various languages files can be found in subdirectories here, for example the Dutch language component can be found in subdirectory nls\nl. Open the strings.txt file, search for the L_UserName_Text string and change its value.

Restart the TMG Firewall service and open Outlook Web App. You’ll see that the logon page has now changed: