A mailbox is part of the postal system
In the United States, a curbside mailbox may belong to a homeowner, but its role as a delivery point is governed by federal law. The rule generally bars anyone from putting unstamped mailable material—such as a flyer or business card—inside a mailbox approved for U.S. Mail. A private company cannot treat it as a convenient free drop box simply because it stands on private property.
The rule is not literally “postal workers only.” Residents can retrieve their mail and put properly stamped outgoing mail in the box. The restriction is on unauthorized delivery into it, not on the owner’s ordinary use.
Why the restriction exists
A mailbox gives the Postal Service a protected route to deliver mail for which postage has been paid. If advertisers or other companies could slip material into the same box without postage, they would gain access to that delivery channel while avoiding its cost. The boundary helps preserve postage as the price of using the mail system and keeps a mailbox from becoming a general-purpose advertising slot.
Federal law specifically addresses knowingly placing mailable matter in a letter box without postage. That is why a flyer left at the door or on a separate newspaper holder is different from one placed inside the mailbox: the mailbox is not just a container on private land; it is a designated part of the postal network.
