And there we have a conflict, do we trust helpdesk or a novice Exchange administrator to have the Full Exchange Administrator permission? probably not. So what can be done?
Luckily there is a way to grant the permission to change permission on mailboxes without being the big Exchange administrator, sort of.
Use Exchange Management Console, drill down to a mailbox database, right click and click properties and select the security tab, add the helpdesk group and select Allow only Change Permission and Administer Information Store.
Change Permission will inherit down to mailboxes. The helpdesk group must also have Active Directory permission to manage user account. This is done with normal AD delegation.
Unfortunately helpdesk group must also have Administer Information Store permission to get what we want. With this right they can also dismount and mount that database.
Two links that help you understand things better.