I can't answer that.
As a postie, I tend to see lots of flits with people owing money here or there. New occupiers move in, but the letters keep coming, and eventually maybe debt collectors.
All I know is that it does no good simply returning letters marked not known and hoping for the best - I mean, even if they got back to the original sender, would you accept that on face value? No, you wouldn't.
So the advice I give people - and it seems to work - is open any letters that look likely, or if you know there is a problem, and then contact them direct to explain the person no longer lives there, and provide some proof that you are not them. Also, for when there might be debt collectors, have some proof to hand to show them, and if they get stroppy or are otherwise a nuisance, threaten to or call the police.
I wouldn't chuck any mail away in these circumstances - just file it, making a note of any action taken. When I said chuck away, I was referring to the ordinary junk stuff.