I am moving about 500 users from FirstClass to Exchange; however, we cannot move everyone at once and would like to do so in waves of 100 users. Is there a way to route mail to different servers based on email alias?
You can configure your Exchange Server to send mails to a "smarthost" if there is no mailbox with the email address in your exchange organization.
you have to set the accepted-domain to "InternalRelay"
Here is a article that describes the necessary steps