The United States Postal Service does not require a return address on most pieces of First-Class Mail. Several exceptions to this postal rule exist, however. Most of these apply to official mail, registered letters and all packages sent via Priority or Express Mail.
The USPS relies on the return address if a delivery is unable to reach its intended recipient. The USPS destroys pieces of mail that do not reach their destination and do not have a legible return address printed on the package. If a sender chooses to use a return address, it should contain both the sender's name and full address.