No, it is not legal to leave door hangers on mailboxes or attach them to the mailbox flag in the United States. According to regulations set by the United States Postal Service (USPS), mailboxes are considered federal property, and their use is reserved exclusively for mail that has been processed by the USPS. This includes the mailbox itself and the mailbox flag.

Title 18, Section 1725 of the United States Code specifies that only authorized USPS mail can be placed in or on a mailbox. Placing unauthorized items, such as door hangers, flyers, or any non-postage-paid material, on a mailbox or its flag is a violation of federal law and can result in fines.

Businesses and individuals looking to distribute door hangers should hang them directly on the doorknob of the door or use other methods of distribution that do not involve the mailbox. This ensures compliance with the law while still allowing for effective local marketing. Other legal alternatives include hand-delivering materials to recipients without using their mailboxes or opting for direct mail services through the USPS, which requires postage but ensures that materials are delivered legally and directly to mailboxes.