Yes, putting flyers in mailboxes without postage is illegal in the United States. This action violates federal law, specifically under the United States Postal Service (USPS) regulations. The USPS has exclusive access to mailboxes, and it’s a federal offense for anyone else to place materials in them without going through the USPS and paying the necessary postage. This rule is outlined in Title 18, Section 1725 of the United States Code, which states that any flyer, brochure, or advertisement placed inside a mailbox without postage is subject to a fine.

The regulation is intended to protect the privacy and integrity of the postal service, ensuring that mailboxes are used only for their intended purpose: the delivery of mail that has been processed through the USPS system. For individuals or businesses looking to distribute flyers, alternatives include door hanger advertisements, direct mail through the USPS where postage is paid, or distributing flyers in public spaces where it is legally permitted, always respecting local ordinances and property rights.