HTTP authenticated access must be set to Integrated Windows Authentication only.

This feature controls the authentication method used to connect to the OWA virtual directories. Ensure this is set to Integrated Windows Authentication only. Anonymous access provides for no access control. Basic Authentication transmits the password in the clear and risks exposure, and the other methods are not recommended by Microsoft for this control. Failure to configure this as per the recommendation may result in unrestricted access to OWA virtual directory, passwords being sent in the clear, and/or the inability to correctly authenticate, depending on which change is made.