This tip explains how to set mailbox rights in the User and Computers MMC under Active Directory on a Windows 2000...
Professional administrator's workstation.
If you install the admin pack on your professional workstation, by default you can't view the Exchange advanced tab in the Active Directory Users and Computers snap-in. It took a while before I sorted out this issue, but it's so simple you never thought the solution would lay therein.
First I thought it would have to do with permissions and rights, but even giving myself full administration rights did not resolve this issue. The real solution is choosing within the snap-in to view the advanced settings. It's as simple as selecting your management domain, clicking View in the action menu, then selecting advanced features.