Exchange 5.5 change sender address

We merged with another company and have changed our Internet domain name. We have set up our Exchange server to accept email from the new domain name as well as the old. We would like to make outgoing email from our office appear as if it were coming from the new name and not the old. Any suggestions on how to do this or knowledge base articles that will help. I’m not even sure what I should be searching for. We run Exchange Server 5.5 and Outlook 2000. Outlook 2000 is set up in Corporateor Workgroup mode and is configured for the Exchange Server service. Thanks for your help.