Intentionally opening another person's mail is a federal offense. Obstructing mail from reaching the intended recipient is also illegal, according to the legal experts at FindLaw.com.
Mistakenly opening mail addressed to someone else does not constitute a crime, however, persons accidentally doing so are advised to notify the United States Post Office immediately.
Mail addressed to previous tenants may be labeled "Return to Sender" or "Wrong Address" and place in the mailbox, as it is a federal crime to intentionally destroy another person's mail.
Additionally, one person may be given verbal or written authorization to open another's mail without penalty.