Not joke. In U.S. it is a federal offense to read someone else's mail.
Someone else's
mail, according to the United States Postal Service. Reading someone else's postal mail can earn you jail time.
Someone else's email or other forms of electronic contact, however, is just a breach of privacy. I don't think you can be thrown into jail for it, but they can take legal action if they really want to put that much time and effort.