Add another domain to existing 2003 AD

Here's the situation.  I have an existing 2003 AD domain w/ Exchange 2003 (xxx@abc.com).  I've added another SMTP domain (xxx@xyz.com) through Recipient Policy and added the appropriate mx records & A records and all works fine.

Question is, users still have to login using their credentials w/ (@abc.com).  I need to be able to add a user and select his/her User logon name: w/ the new email address domain (xxx@xyz.com).

Thanks in advance,
Art