Certifie Mail is a type of Special Service mail offered by the United States Postal Service which allows the sender proof of mailing, as well as proof of delivery. Certified Mail also provides the sender with a copy of the recipient's signature, which is obtained at the time of delivery, by the Postal Service. Certified Mail is only available for Priority Mail and First Class Mail letters mailed within the United States and its territories (including APOs and FPOs). Each piece of certified mail is assigned a unique tracking number. This number serves as both an official record of mailing and receipt of the item by the Postal Service.