I have been asked by one client of mine to do a redirection to their mail.domain.com to go directly to the Owa page but after applying the redirection configuration I faced an issue.

Whenever I try to go to the OWA page using only the FQDN mail.domain.com I get a 401 unauthorized access page.

Resolution:

The solution was to add authenticated users to wwwroot with full permission and restart the IIS with noforce parameter.