Would be useful when moving a domain from an external server to your own server to allow for accounts to be created for the domain prior to the actual move. Currently, this gives two issues. First, you have to use a forwarding workaround (on each account) to continue to deliver mail to the existing server and second, during the time you're creating the accounts, any emails sent locally to un-configured accounts will be returned to the sender with a mailbox unknown error. How much easier it would be if there was a flag that denoted that emails for this domain should continue to be sent externally until you need to make the domain "live".