Items that are illegal to put through a mailbox are Newspapers, except for Sundays, flyers, menus, advertisements, gifts, sale bills or like matter or anything else that does contain postage and hasn't went through the USPS.
It's illegal to put anything inside a mailbox that is not official U.S. Mail that has proper postage, which includes menus, flyers and other unauthorized items.
Putting unauthorized items in mailboxes can prevent mail carriers from delivering official mail and can clog the mailbox.
Violators can face fines up to $10,000.00
Any commercial or even non commercial advertisements or literature without proper postage cannot be placed in a mailbox.
And anything else that is placed in a mailbox without postage, including gifts is also illegal.
And you cannot place any handwritten notes or cards in another person's mailbox without proper postage.
Although Newspapers can be placed in a mailbox on Sundays, as Sunday is a non delivery day for USPS.
And some community mailboxes also have attached receptacles for newspapers, but these must not touch the mailbox or interfere with normal mail delivery.
Items that are illegal to send through the mail are firearms, fireworks, ammunition, controlled drugs and narcotics, Marijuana, whether it's medical use or otherwise, dangerous goods, lithium batteries, mercury, nail polish, paint, aerosol spray dispensers, live animals, cigarettes, alcohol and explosives as well as gasoline or any other flammable liquids.