Correspondence or documents sent by the U.S. post office and officially marked to whom sent, showing date and address of delivery. This type of mail service is used often by medical offices for collection purposes and important letters (e.g., relocation of a medical practice, closure, dismissal of a patient). When certified mail is received, the receiver must sign a receipt of delivery and the sender is provided with a receipt of the delivery. This receipt remains on file at the receiver’s post office for 2 years.