Exchange 2000 Mail Box Rights

I have an Exchange 2000 native system that was an upgrade from a 5.5 domain. Windows 2000 is still mixed domain. I have users that cannot change the permissions to allow other access to their calendar because there are old users “NT User S1-586-?”listed. Every time you try to change it says, “The Modified permissions could not be saved. The client operation failed.” How do I get rid of these old entries and modify the permissions. Are there any 3rd party utilities to modify permissions? HELP!!!