The U.S. Code for crimes and criminal procedure prohibits the placement of unstamped flyers in any mailbox. Title 18, section 1725 states that any person who knowingly deposits "mailable matter" without postage in an established letter box shall be subject to a fine.
The Government Accountability Office reports that fines may be as high as $5,000 per occurrence for individuals and $10,000 per occurrence for organizations.
This law is commonly known as the "mailbox restriction."
The restriction also includes anything placed upon, supported by, attached to, hung from, or inserted into a mail receptacle.
Circulars and sale bills are specifically prohibited in the document (see below).
Mail slots in residential doors are not covered under postal policy or the statute, because they provide direct access into the home itself.
Non-lockable bins or troughs used with apartment house mailboxes” are also excluded.
Customer Mail Receptacles (USPS)
3.1.1 Authorized Depository
Except as excluded by 3.1.2, every letterbox or other receptacle intended or used for the receipt or delivery of mail on any city delivery route, rural delivery route, highway contract route, or other mail route is designated an authorized depository for mail within the meaning of 18 USC 1702, 1705, 1708, and 1725.
Door slots and nonlockable bins or troughs used with apartment house mailboxes are not letterboxes within the meaning of 18 USC 1725 and are not private mail receptacles for the standards for mailable matter not bearing postage found in or on private mail receptacles. The post or other support is not part of the receptacle.
3.1.3 Use for Mail
Except under 3.2.11, the receptacles described in 3.1.1 may be used only for matter bearing postage. Other than as permitted by 3.2.10, or 3.2.11, no part of a mail receptacle may be used to deliver any matter not bearing postage, including items or matter placed upon, supported by, attached to, hung from, or inserted into a mail receptacle. Any mailable matter not bearing postage and found as described above is subject to the same postage as would be paid if it were carried by mail.