When you set full access permissions with the Exchange 2010 management console on a mailbox,the user which has be giving full access right now there will filed a new property. 
The property is MSExchDelegateListLink. You can check it with adsiedit. 
When you open now Outlook 2010 you can now see that Wardtest is automatilly Added
And additional mailboxes is empty 
When you migrating from Exchange 2007 to Exchange 2010 and upgraded the Clients to Outlook 2010
Use dis script to set the MSExchDelegateListLink object automatically. Test I first. It’s your own risk.
$a=get-MailboxPermission * | where {$_.IsInherited –eq $False –and $_.accessrights –eq “FullAccess”}
$a | remove-MailboxPermission –confirm:$false
$a | add-MailboxPermission
Save the scripts to MSExchDelegateListLink.ps1
It works great 
Source