Someone whose mail has been intercepted and opened by another person should report the crime to the U.S. Postal Inspection Service, which is the federal law enforcement and security arm of the U.S. Postal Service. If the situation is urgent, the USPS recommends contacting the nearest law enforcement agency.
One area the U.S. Postal Inspection Service enforces is the destruction, obstruction and delay of mail. Specifically, opening someone else's mail falls under 18 U.S. Code 1702, which is obstruction of correspondence. The law stipulates that anyone who takes or opens a piece of mail before it is delivered to the person to whom it is addressed, with the intention of obstructing the correspondence or prying into the business or secrets of someone else, is guilty of criminal behavior. The penalty is a fine, imprisonment of up to five years or both. If the person does not open the mail but hides or destroys it, the penalty is the same.
Victims can send mail theft or obstruction of correspondence complaints by means of an email link on the U.S. Postal Inspection Service website. Alternatively, a form can be downloaded, printed and mailed to an address provided on the site. The victim should share all relevant information; the Postal Inspection Service states that the information may be shared with other agencies involved in postal crime prevention and detection.