[rootusrootus]

For anyone else who runs across this, in the US you want to also put a line through the bar code at the bottom of the letter, so it cannot be scanned. Once a piece of mail gets that code, the post office stops reading anything else on the letter and just delivers the mail to where that code says it goes. So you can toss it back in an outgoing slot with 'return to sender' on it as many times as you like, and they'll just return it to you. Until you get lucky and the mail carrier sees it when gathering up the outgoing mail, and helpfully obscures that barcode for you.