In the UK the Post Office attempts to deliver letters to the addressee, not just the address. One side effect of this is that you can re-write the address on an unopened letter, stick it back in a letter box, and it will be forwarded on.
This also seems to work for junk mail. I didn't like getting a deluge of paper crap for umpteen former tenants of this property, so I systematically sent it all back. Happily after about a couple of months it almost entirely stopped.
But it hasn't all stopped, and in particular it seems that mail comes for the previous person named on the electoral register. One piece has come franked not by the Post Office, but by "UKMail Businesspost". Yes, mail delivery deregulation now means that there are private operators sending junk mail too, although I believe that currently they still use the Post Office's delivery staff for that last mile of copper. (ish).
So I thought "right. What's the return address?" Then I thought "But I can't put it in the letterbox, as it has no valid Post Office postage." Then I thought "Why do I care? It doesn't get back to me once it goes in the letterbox, and if the Post Office change the sender the standard cost for delivering un-stamped mail, that's just fine by me. Puts the marketer's costs up. Might reduce the flood slightly."