Most users don’t even know their Username and Password so when presented with Domain\user name this just adds to the confusion. You have 2 options for this. You can change it to the users UPN by using the following command:
Get-OwaVirtualDirectory | Set-OwaVirtualDirectory -LogonFormat PrincipalName
Problem being now that the UPN has to match the users email address as that is actually what their credentials are not the email address attribute in Exchange. This is usually ideal for organisations that host a multi tenanted Exchange environment.
I prefer just the Username. This time just use the ECP:
Select the User name only option and select your domain. Reset IIS and then refresh your browser:
Much better for users.