Automatic warning for people sending to 'old' domain

Such scenarios are usually addressed by adding an Automatic reply to each and evert mailbox you expect people will still be sending messages too. A centralized solution is also possible, but you will loose the ability to alert the sender for the "best match" new address.

Another thing to have in mind is that Automatic replies only trigger once per sender. If you want this warning to be displayed on each mail received, you can use Inbox rule instead.