Is it legal to put flyers or business cards on mailboxes?

No. Under U.S. federal law, mailboxes are reserved exclusively for U.S. Postal Service mail, so placing flyers, business cards, or any other item in or on a mailbox without proper postage is illegal.

• What is prohibited: putting anything in the box, on the box, or under the mailbox flag without going through USPS.
• Penalties: violations can carry fines, up to roughly $5,000 for individuals and up to about $10,000 for organizations.
• Legal alternatives: place pieces on doors or windshields, or hand them out directly in person.

The one fully compliant way to land in the mailbox is to mail the piece with postage. Direct mail reaches the same households legally and with delivery you can rely on.