All Letters have an Originating Postmark, or a Stamp (not like the sticky postage stamp), so Just write on the Envelope, "No longer at this address" and give it back to the Postman/Woman when you next see them. Eventually, it will get back to the Originating Post office sorting place in whatever County it was sent from, from that it'll probably get returned to the original sender, they'll probably have opened the card by now and found that its from some Little old Lady called Doris that still thinks that her dear old friend who died 100 years ago, still lives at Bungles house.