Multiple Email Domain Addresses

Hi!!

Exchange 2000. Windows 2000. Office 2000. SP’d to latest.

Problem: I have a company “xyz” that owns another company “abc”.
I have a small group of users who work for both of the companies but the staff cannot allow the clients to know that they work for both companies.
This means when emails are sent out they must be sent out with the correct domain address eg. user@abc.com or user@xyz.com

At present the only way i can find to allow the users to change the domain they are sending from is for me to manually change the default email address in Active Directory.
This needless to say is annoying and a waste of my time and the users time.

Question: How can i allow the user to change their default email address themselves?
Thanks

Vel