Exchange 2000 with diffrenet domain name

My question about Exchange 2000 server, but I will explained my network.
We have an windows 2000 domain named domainA which is not registered.
On another network named domainB which is registered we have exchange 2000 server.
The e-mails for users in domainA are on exchange server on domainB (user@domainB)
I need to setup exchange server 2000 in domainA and move the users e-mail to exchange server 2000 in domainB but without changing the user e-mail. It must remain as user@domainB.
How can I do this?
Thanks