18 U.S.C. § 1463

All matter otherwise mailable by law, upon the envelope or outside cover or wrapper of which, and all postal cards upon which, any delineations, epithets, terms, or language of an indecent, lewd, lascivious, or obscene character are written or printed or otherwise impressed or apparent, are nonmailable matter, and shall not be conveyed in the mails nor delivered from any post office nor by any letter carrier, and shall be withdrawn from the mails under such regulations as the Postal Service shall prescribe. Whoever knowingly deposits for mailing or delivery, anything declared by this section to be nonmailable matter, or knowingly takes the same from the mails for the purpose of circulating or disposing of or aiding in the circulation or disposition of the same, shall be fined under this title or imprisoned not more than five years, or both.