Question : How do I transition mailboxes from exchange 2003 to exchange 2010 when both exchange servers are in different domains without causing an outage

I'm trying to move all my exchange user mailboxes from my old windows domain [DomainAa] to my new domain [Domain B] without causing an outage of email service.

[Domain A] uses exchange 2003 and [Domain B] uses exchange 2010.   All mail is presently delivered to the exchange 2003 server on [Domain A].

I found a script that can migrate the mailboxes over to [Domainb B].  However once the mailbox is moved all future email deliverd to the exchage 2003 server on [Domain A] for the migrated mailbox is undeliverable.

Is there a way to automatically forward email from the old exchange server on [Domain A] to the new exchange server on [Domain B] when the mailbox is no longer located on [Domain A]?

I have to migrate 1000 users and don't want any mail to be undelivered during the transition.

Answer : How do I transition mailboxes from exchange 2003 to exchange 2010 when both exchange servers are in different domains without causing an outage

I would think if you have all new mail delivered to the new Domain B Exchange Server at a cut-off point, and have users log into the new Domain to get their mail, promising them that within 'x' time, their old mail will show up...

Then you begin the export and reimport of their mail on your schedule (maybe with VIPs first on the list)