domain change and auto email setup

Our companies domain is getting ready to switch over to the company that bought us. When email comes in on the old domain we are wanting a auto email to send back to that sender stating that the person’s new email address is on the new domain. We still want to process and deliver the inbound email to the person. I know we can setup alais under Exchange to take care of that part. But it is the auto email part that I do not know how to do. Anyone out there ever run into a similiar problem? Can this even be done?