Move mailboxes to different domain

I have a windows 2000 domain running windows 2000 servers and exchange 2000 with a 123.com domain name. My users have their email set up as jsmith@123.com. We purchased a small company the has NT 4.0 server and exchange 5.5. They have email as John@456.com. I need to forward the mail that comes into their acct’s John@456.com to their new acct’s at Jsmith@123.com from their Exchange 5.5 server to my Exchange 2000 server in a different domain. Please be specific.