This KB will show you how to give admin user full access permissions to all mailboxes In the organization.
The script will work on Exchange Server 2007, 2010, 2013 and 2016.
get-mailbox -ResultSize unlimited | Add-MailboxPermission -User administrator -AccessRights fullaccess
One done you will see the permission that were added
To reverse the permissions use the same script just use Remove-MailboxPermission