18 U.S.C. Section 876. Mailing Threatening Communications [Extortion] . . . (b) Whoever, with intent to extort from any person any money or other thing of value, so deposits, or causes to be delivered, as aforesaid, any communication containing any threat to kidnap any person or any threat to injure the person of the addressee or of another, shall be fined under this title or imprisoned not more than twenty years, or both.