adding second domain to e-mail accounts

My company is goin to be changing our Domain name within the next 12 months to reflect changes in the organization. We have purchased the new Domain name and have set up our web services to allow users to access the site. The major problem I am having is allowing my Echange 2000 server to recieve mail under the old address and the new address. I have added a second SMTP entry into the users account and it sends fine, I can recieve if the old account is removed. But when I add it back (as a secondary address) the new Domain extension will not recieve incoming mail. Does anyone know how to route the incoming mail to the same box with either the old or new Domain Name? I hope I explained this well enough.