What does this icon mean for the user in the GAL?

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.
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