Even though you bought and maintain your own mailbox, it is deemed to belong to the United States Postal Service. This means that tampering with the mailbox is a federal offense. "Mail tampering" has a broad meaning and includes any act that might obstruct the delivery of mail, from vandalizing the mailbox to delivering a few innocent flyers.
It is a federal offense to tamper with a mailbox or the items in it. That includes putting things into a mailbox as well as taking things out of it. There is a significant penalty for violating this law.
Putting Items Into a Mailbox
Only authorized USPS letter carriers may insert mail into a residential mailbox. The USPS Domestic Mail Manual is very clear that mailboxes are not to be used for anything other than official mail with postage attached, so you can't go around the neighborhood delivering flyers. It's prohibited to place anything on or around the mailbox, too, so you can't use them for placing stickers. Willfully violating these rules is the federal offense of "mailbox restriction," with fines as high as $5,000 per occurrence.
Taking Items Out of a Mailbox
It's a federal crime to steal someone's mailbox, to steal mail from a mailbox, or to snatch mail items that are in the process of delivery. It's a separate offense to take a letter in order to pry into the affairs of another person, even if you only look at the return address momentarily. Whether you're charged with mail theft or obstruction of correspondence, you're potentially looking at fines of up to $250,000 and five years' incarceration. The only exceptions are when you have the addressee's permission to handle the mail or you're legitimately dealing with a deceased person's affairs.
Vandalizing a Mailbox
Unsurprisingly, vandalizing a mailbox is also a federal crime. Vandalism in this context means any type of destruction or property damage to the mailbox itself or inserting something into the mailbox with the intention of defacing or destroying the mail inside. If you're found guilty of mailbox vandalism, you can expect a fine up to $250,000 and up to three years in a federal prison for each willful act.
Protecting Your Mailbox
While a mailbox is considered federal property, it's up to each homeowner to maintain his own mailbox. If you're worried about mailbox tampering, USPS recommends that you obtain Label 33 from the post office and affix it to your mailbox. The sticker carries the warning: "Willful damage to mailboxes or theft of mail are federal crimes (felonies) punishable by fine or imprisonment or both." If you suspect that your mail has been tampered with, immediately report it to your post office.