Although your IIS 6.0 server is a member of a Windows 2000 domain and is correctly configured for Digest authentication as well as subauthentication, logging on with a user principal name (UPN) in the form username@domain.com fails. This failure occurs because Digest authentication accepts domain logons in two formats only. If a user logs on to a server in the same domain where the user's account resides, username alone is sufficient for authentication. For logons with a user account from another domain, use the domain\username format