A certified letter, also known as certified mail, incorporates a unique delivery process that requires proof of delivery or receipt. The sender retains a mailing receipt, and the post office maintains a delivery record. It is commonly used for important mail, such as legal notifications.
Senders of a certified letter can pay an additional fee for a return receipt that also provides them with proof of delivery. A person can send a certified letter via First-Class Mail and Priority Mail. The U.S. Postal Service offers an online certified mail service which handles the entire process, including tracking and confirmation, on the USPS site.