Auto Reply based on "TO:" address

Hi, This seems like a common issue but after searching through the forums, nothing seems readily available. I have two domains, one is old and we do not control the email accounts or DNS ("olddomain.com"). Our new domain is functional under CPanel and emails are working. Our old hosting company is redirecting emails sent to the old user accounts to our new user accounts. For example, user@olddomain.com is forwarding to user@newdomain.com. We have published our new email addresses, sent out notices, etc, etc but many suppliers and customers are still using our old email addresses. I was looking at somehow setting up User Filters and piping to a program, based on the "To:" field equals "user@olddomain.com" and send out a reply "Please update your addressbook. My new address is user@newdomain.com". Probably best to include a frequency of once a day or less frequently. I also need the email to come from "user@newdomain.com" so I don't think a Global filter would work. The other option is to try and use EXIM ROUTES and TRANSPORTS with a vacation message with some extra logic but that seems like a more complex option. Has anyone any suggestions?