I recently changed the authentication type for logging into OWA to allow logging on using email address instead of domain\username (forms based with User Principle Name). This works fine and users can login using email address.
However, any newly created users can only login using domain\username or OWA states the username or password is incorrect. But if they use domain\username with same password it works fine. All existing users are fine to login using their email address.
Even after they have successfully logged into OWA for the first time and reset their password, the above remains true for only new users.