19 CFR § 145.51 - Articles prohibited by section 305, Tariff Act of 1930.
(a) Types of articles. Various articles, as described in section 305, Tariff Act of 1930, as amended (19 U.S.C. 1305), and in part 12 of this chapter, are prohibited from importation. This prohibition includes the following types of articles:
(1) Obscene matter;
(2) Articles for causing unlawful abortion (see § 145.52 for the treatment of literature pertaining to such articles);
(3) Matter advocating treason or insurrection against the United States or forcible resistance to any law of the United States;
(4) Matter containing any threat to take the life of or inflict bodily harm upon any person in the United States; and
(5) Lottery matter, except any lottery ticket, printed paper that may be used as a lottery ticket, or advertisement of any lottery, that is printed in Canada for use in connection with a lottery conducted in the United States.
(b) Disposition of articles. Mail found to contain lottery matter shall be disposed of by the Postal Service under the postal laws and regulations. Mail found to contain any of the other prohibited articles described in paragraphs (a)(1) through (a)(4) of this section shall be given appropriate treatment by Customs under the Customs laws and regulations (see § 12.40 of this chapter).