If there is no postage on it, it will be returned if there is a return address. No return address, USPS will collect the 11 cents shortage from the addressee. If the addressee refuses and there is no return address, the letter goes to a Mail Recovery Center, where it will be opened to see if there is a return address inside. If not, it will be shredded after removing anything of value, which USPS can keep. If there is a return address, it will be returned to you and you will have to pay the 11 cents if you want it back.