Getting rid of the old mailbox database? Before you delete the old one, you have to move the system related mailboxes to the new one. This is how you do it on Exchange Server 2010;
Go to Powershell on your Exchange Server and enter:
- Get-Mailbox -database YOUROLDDBNAME -Arbitration
You can see the entry in the red box.
Get-Mailbox -database YOUROLDDBNAME -Arbitration | New-MoveRequest -TargetDatabase YOURNEWDBNAME