18 U.S.C. § 1461 : US Code - Section 1461: Mailing obscene or crime-inciting matter

      Every obscene, lewd, lascivious, indecent, filthy or vile
    article, matter, thing, device, or substance; and - 
      Every article or thing designed, adapted, or intended for
    producing abortion, or for any indecent or immoral use; and
      Every article, instrument, substance, drug, medicine, or thing
    which is advertised or described in a manner calculated to lead
    another to use or apply it for producing abortion, or for any
    indecent or immoral purpose; and
      Every written or printed card, letter, circular, book, pamphlet,
    advertisement, or notice of any kind giving information, directly
    or indirectly, where, or how, or from whom, or by what means any of
    such mentioned matters, articles, or things may be obtained or
    made, or where or by whom any act or operation of any kind for the
    procuring or producing of abortion will be done or performed, or
    how or by what means abortion may be produced, whether sealed or
    unsealed; and
      Every paper, writing, advertisement, or representation that any
    article, instrument, substance, drug, medicine, or thing may, or
    can, be used or applied for producing abortion, or for any indecent
    or immoral purpose; and
      Every description calculated to induce or incite a person to so
    use or apply any such article, instrument, substance, drug,
    medicine, or thing - 
      Is declared to be nonmailable matter and shall not be conveyed in
    the mails or delivered from any post office or by any letter
    carrier.
      Whoever knowingly uses the mails for the mailing, carriage in the
    mails, or delivery of anything declared by this section or section
    3001(e) of title 39 to be nonmailable, or knowingly causes to be
    delivered by mail according to the direction thereon, or at the
    place at which it is directed to be delivered by the person to whom
    it is addressed, or knowingly takes any such thing from the mails
    for the purpose of circulating or disposing thereof, or of aiding
    in the circulation or disposition thereof, shall be fined under
    this title or imprisoned not more than five years, or both, for the
    first such offense, and shall be fined under this title or
    imprisoned not more than ten years, or both, for each such offense
    thereafter.
      The term "indecent", as used in this section includes matter of a
    character tending to incite arson, murder, or assassination.