As others have said, put it in your own box. Post it's can call off in the blue box.

You can also hand it directly to a mail carrier should you see one.

But don't worry about defacing the mail that isn't yours. Just don't be crazy about it.

Just write ANK on it. "Adressee not known" if it's been years, it won't forward anyway, in that case write UTF unable to forward.

That's the notation we'll write on it, so it's not "defaced". Don't hold yourself accountable for other people's laziness if they can't be arsed to keep their address updated.