Default Permissions -Exchng 2K mailboxes

For some odd reason any user can open another persons Mailbox contents by choosing File Open other user's folder, from within Microsoft Outlook.

In addition, if you add another user's mailbox to the Outlook Profile he/she can see all of the contents, without rights being given in the owner's Outlook profile. I would like to correct this problem. Even the Administrator can open other user's mailboxes, which I know was not the default when this Exchange 2000 server was first set up. Somethingchanged on the Server, but I don't know what. I feel if I can find out the default settings I can compare everything and figure this out. Please help! Thanks.