routing of emails into 2 internet domain

I have an NT Server 4 running a DNS and an Exchange 5.5. Currently have a domain call abc.com with exchange mailbox user@abc.com. I would like to add a new domain xyz.com. And move users mailboxes to the new domain xyz.com. So users will be using user@xyz.com. In the future the old internet domain will be removed. What is the best way in configuring this. Thank you in advance.