18 U.S.C. § 1702 : US Code - Section 1702: Obstruction of correspondence

      Whoever takes any letter, postal card, or package out of any post
    office or any authorized depository for mail matter, or from any
    letter or mail carrier, or which has been in any post office or
    authorized depository, or in the custody of any letter or mail
    carrier, before it has been delivered to the person to whom it was
    directed, with design to obstruct the correspondence, or to pry
    into the business or secrets of another, or opens, secretes,
    embezzles, or destroys the same, shall be fined under this title or
    imprisoned not more than five years, or both.