18 U.S.C. § 953 : US Code - Section 953: Private correspondence with foreign governments

      Any citizen of the United States, wherever he may be, who,
    without authority of the United States, directly or indirectly
    commences or carries on any correspondence or intercourse with any
    foreign government or any officer or agent thereof, with intent to
    influence the measures or conduct of any foreign government or of
    any officer or agent thereof, in relation to any disputes or
    controversies with the United States, or to defeat the measures of
    the United States, shall be fined under this title or imprisoned
    not more than three years, or both.
      This section shall not abridge the right of a citizen to apply,
    himself or his agent, to any foreign government or the agents
    thereof for redress of any injury which he may have sustained from
    such government or any of its agents or subjects.