MikeL2: You didn't way what country you're in, but if this were the US, I'd bet the problem is with the barcode.

Nowadays, the USPS uses machines that read the address and print a barcode within a half inch of the bottom edge of the envelope. Once a barcode is printed on the envelope, all subsequent sorting will be done according to the barcode, and the printed address will be ignored.

Once in a while the machine misfeeds and sends two envelopes through at once, one overlapping the other. Then the machine might read the address off the top envelope and print the barcode on the bottom one, as if it were the same envelope.

Some kinds of mail arrive at the post office with a barcode already printed on it. Those envelopes will bypass the OCR/barcoder and be sorted according to the existing barcode.

So if your mystery envelope has an incorrect barcode, for whatever reason, it will keep coming back to you as long as that barcode remains unchanged. To make sure it doesn't come back again, you must make the existing barcode illegible.

I don't know whether other countries use the same procedures or equipment.