Grant a User Full Access to All Mailboxes
open exchange management shell.
Get-Mailbox | Add-MailboxPermission -User 'yourdomain.com\theAdminuser' -AccessRights 'FullAccess'
open exchange management shell.
Get-Mailbox | Add-MailboxPermission -User 'yourdomain.com\theAdminuser' -AccessRights 'FullAccess'