This is an Exchange 2007 environment. A few users have a "disabled" looking icon in the GAL. The accounts look fine and the users themselves are not complaining. The problem arises when someone wants to share a calendar with that user. Then you get an error message: "One or more users cannot be added to the folder access list. Non-local users cannot be given rights on this server". The user accounts look fine so the non-local appears to be a red herring.