Most people are aware that removing mail from a mailbox is a crime. However, the rules regarding opening another person's mailbox or putting material inside may be unclear to some.
The United States Postal Service (USPS) has no specific guidelines against opening someone's mailbox, but there are laws against removing mail or placing items in the mailbox. The USPS mandates that only items with the required amount of postage affixed may be placed inside of a mailbox.
Areas Surrounding Mailboxes
Not only is it illegal to place unauthorized items inside of a mailbox, it is also a crime to place them around the mailbox. The USPS states that "no part of a mail receptacle may be used to deliver any matter not bearing postage." This means that items cannot be placed on, hung from or rested against the mailbox or post.
If your original intent of opening the mailbox was to place an item inside, there are exceptions that allow you to leave items. Doorslots and unlockable bins and troughs used in apartment buildings are exempt from the regulations and non-postage items may be placed inside.