USPS has to deliver a lot of mails on daily basics; mistakes do happen once in a while; it is either the mail has not come to the right address, or it is addressed to someone that is not living there.
Before returning the mail for someone not living there, write on the envelope, “not this address.” Most times, mails are usually delivered to the wrong address.
Here are somethings you have to do: put back the item in your mailbox or hand the item back to the delivery person if you prefer, don’t mark or write anything on the item.
Finally, for unwanted mail addressed to you, you have to write “refuse” on the envelop, and if you have opened it, repackage the item with new postage.