Assume that you configure a full domain sign-in format (Domain\user) for Outlook Web App by running the Set-OwaVirtualDirectory cmdlet in Exchange Management Shell (EMS) in a Microsoft Exchange Server 2013 environment. When you try to log on to Outlook Web App by using a mobile device, such as an Apple iOS device or Windows Phone 8, the logon page of Outlook Web App prompts you to enter your email address instead of entering credentials in the "Domain\user" format.
This issue occurs because Outlook Web App labels are hardcoded for mobile devices. Therefore, the customized sign-in format does not work.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about the Set-OwaVirtualDirectory cmdlet, go to the following Microsoft website: