Sophomore Candy Chretien encountered problems this year when someone else's mail kept showing up in her mailbox. I actually went to the mailroom about a week ago and used the key to see if it was still my mail, but it's some girl's mail now. There was just a sign in the MUB, so I turned it in the last day of school. I would assume you would be charged if you didn't. I understand she was worried about a privacy issue, but most people do return mail if, for some reason, it ends up in the wrong box.
Most people are honest about it.