Exchange 2003 Server added to an Exchange 5.5 Site/Organization

[Orbital911]

I need information on how to configure an Exchange 2003 Server when added to an existing Exchange 5.5 site. Upon adding the E2K3 server to the domain, we will only be moving 5-10 mailboxes at a time, allowing them to reside on the new server for approximately a week, before moving additional mailboxes to ensure the new server is stable and operates correctly. While running the original Exchange 5.5 server (approx. 150 mailboxes) and the new Exchange 2003 Server (will only have 5-10 mailboxes moved per week until migration is finished) how will the 5.5 server know when to route mail to the 2003 server if the mailbox has been moved from the 5.5 to 2003 server? What configurations or connections will need to be setup between the 2 servers? Any help is immensely appreciated. Thanks!