What Is The Federal Law For Opening Mail Not Addressed To You?

http://thelawdictionary.org/article/what-is-the-federal-law-for-opening-mail-not-addressed-to-you/

Although you may have opened someone else's mail unintentionally, what you do with it afterward is what really counts. Toss the mail in the garbage, and you have intentionally obstructed the delivery of that correspondence. That is a crime, and there may be consequences. The best practice is to write Return to Sender or note Wrong Address on the envelope and pop it into a mailbox. This way the letter can still eventually reach the intended recipient.

Law Dictionary: What Is The Federal Law For Opening Mail Not Addressed To You?