Note You must have the appropriate Exchange administrative permissions to do this. Start Active Directory Users and Computers. On the View menu, ensure that the Advanced Features check box is selected. Double-click the user whose mailbox you want to give permissions to. On the Exchange Advanced tab, click Mailbox Rights. Click Add, click the user.
We recently had a client who had a number of PST files open in their outlook, and some time after installing Sophos found that they could not open the PSTs.-