If a person intentionally opens mail that does not belong to him, it is considered a federal offense. However, if mail is opened by mistake, no crime has been committed, explains J. Hirby for The Law Dictionary.
According to federal law, it is illegal for a person to intentionally prevent a letter from being delivered to the intended recipient. If an individual fails to inform the postal service he is in possession of mail that does not belong to him, this is also a federal offense, notes Hirby. If a person has mail that is not his, he can write "Incorrect Address" or "Return to Sender" on the mail and place it in the nearest mailbox.