Every obscene, lewd, or lascivious, and every filthy book, pamphlet writing or other publication of an indecent character is hereby declared nonmailable matter
and shall not be conveyed in the mails or delivered from any post office or by any letter carrier.
Part IV of the Postal Reorganization Act of 1970 (hereinafter "Postal Act"), determines what is nonmailable matter
.(86) Nonmailable matter
is "[m]atter the deposit of which in the mails is punishable under section 1302, 1341, 1342, 1461, 1463, 1714,(87) 1715,(88) 1717, 1718,(89) or 1738 of title 18.