If they can see and modify the other calendars, then they have permissions for them. Check the permissions in the parent container(s) in AD.

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Sue Mosher, Outlook MVP
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Microsoft Outlook Programming - Jumpstart for
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>I have an Exhcange 2003 server, all users are on office 2003 with outlook,
> the problem is that no calendars are being shared, yet some systems are able
> to view any users calendars, even though they have no rights to view, thay
> can even modify the others users calendars.