Yeah; put it in a post box, period.

If they have a personal (outgoing) mailbox that they are putting just the one piece of mail into, the mail person can skim over it, see the marker, and make decisions.

If it's a post box that is filled with a bunch of people's outgoing mail, no postal worker is going to read over it that way. Further, if you've blacked out the 'to' address with a marker (as I do), and written 'Return to Sender', they then are faced with either returning it to sender...or treating it as a dead letter that is undeliverable. They won't do the latter if there is a sender they can return it to, ergo...back it goes.