If you printed up a flier and walked around shoving them into people’s mailboxes, that would be illegal because it is misuse of postal systems/property.
But what if someone else’s mail gets delivered to you by mistake? Can you just put it in their mailbox? Can anyone just use mailboxes as long as they are being used rightfully and lawfully, or do you have to be some sort of “Certified Deputy of the Postmaster General”?
I know you can just write “delivered to wrong address” on the envelope, but sometimes it is easier to just bring it a few doors down.
Assume for the sake of the question that the mailbox is on or near a public road (accessible without trespassing).