MS Exchange 2007 or more recent

1. Open the Microsoft Exchange Management Shell as an administrator.

2. Run the following commands:

   Add-MailboxPermission -user USERNAME -AccessRights FullAccess -InheritanceType All MAILBOX  Add-ADPermission -user USERNAME -ExtendedRights Send-As MAILBOX  Add-ADPermission -user USERNAME -ExtendedRights Receive-As MAILBOX

   substituting USERNAME and MAILBOX with your own username and the name of the user of the target mailbox.
   
   

MS Exchange 2003

1. Click Start > Administrative Tools > Active Directory Users and Computers.
   
   

2. Under Users, right click on the user whose mailbox you wish to restore to, and click Properties.
   

   
   
   

3. In Exchange Advanced, click Mailbox Rights.

   
   
   

4. In the Permissions dialog, click Advanced, select your user name in the list, and click Edit.

   If your user name does not appear on the list, then click Add, use the dialog to select your user name, then click OK.

   
   
   

5. Check Full mailbox access - Allow , and click OK.