Exchange 5.5 change sender address

Our company merged with another and have taken on a new domain name on the Internet. We have Exchange set up to receive mail with the new name. We would like to have all outgoing email from our office appear as the new name and not the old. Any ideas on how this can be accomplished or knowledgebase articles that will help, I don’t even know what to search for. We use Exchange Server 5.5 and Outlook 2000. The Outlook 2000 clients are configured in Corporate or Workgroup mode with the Microsoft Exchange Server service. Thanks for you help.