I have found documentation showing how to do this, but not EXACTLY how I need to have this done.

My company recently purchase a new domain <> and would like to migrate all the users from <> to the new domain. However, they want the users to still get mail sent to <> and <> but use <> as the primary e-mail they send and recieve from. Is it possible to do this and have all the e-mail sent to put in the inbox of @new_domain? or am I going to have to map the inbox of the old domain?