If you're suffering from "loop mail," the barcode is the most likely culprit.
Sick of receiving mail for that guy who lived in your apartment four years ago? In most cases, there's a simple solution to this perennial renter's woe: Write "RETURN TO SENDER" or "NOT AT THIS ADDRESS" clearly on the envelope and put it back in the mail. Or to be more specific, your best bet is actually to hand it off to your letter carrier in person; if you drop it into a blue collection box, it might get lost in the pile without a postal worker seeing your note.
But let's say you diligently wrote "RETURN TO SENDER" on the envelope and put it back in the mail and it still boomerangs back to your mailbox a few days later. In that case, you might be suffering from "loop mail," an unfortunate side effect of USPS's automated "Intelligent Mail" barcode system.
USPS prints a barcode on each piece of mail, and that barcode is specific to the destination address. Whenever the sorting system reads that barcode, it will direct mail to that address—no matter what you've written on the outside. This process is completely automated, so once that envelope is in the hands of a letter carrier, chances are that he or she never saw the RTS note. "They don’t usually see individual pieces of mail," says USPS spokesperson Sue Brennan. "They see bundles of mail for individual addresses."
To stop loop mail, you need to cross out the barcode at the bottom of the envelope (not the one in the plastic window). This will alert the system that the mail is undeliverable and take it out of circulation. If, after all that, you're still receiving loop mail, Brennan recommends going to your local post office and asking to speak to a station manager. Explain that you've been having a recurring problem with First-Class Mail that needs to be returned, and he or she will be able to tell you how to put a stop to it.