How to assign users or groups full access to other user mailboxes

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 or group who you want to have access to this mailbox, and then click OK.
Be sure that the user or group is selected in the Name box. In the Permissions list, click Allow next to Full Access, and then click OK.

How to assign users or groups full access to other user mailboxes in Exchange 2003