39 U.S.C. § 3204

(a) Except as otherwise provided in this section or section 3220(a) of this title , an officer, executive department, or independent establishment of the Government of the United States may not mail, as penalty mail, any article or document unless— (1) a request therefor has been previously received by the department or establishment; or (2) its mailings is required by law. (b) Subsection (a) of this section does not prohibit the mailing, as penalty mail, by an officer, executive department, or independent agency of— (1) enclosures reasonably related to the subject matter of official correspondence; (2) informational releases relating to the census of the United States and authorized by title 13; (3) matter concerning the sale of Government securities; (4) forms, blanks, and copies of statutes, rules, regulations, instructions, administrative orders, and interpretations necessary in the administration of the department or establishment; (5) agricultural bulletins; (6) lists of public documents offered for sale by the Superintendent of Documents; (7) announcements of the publication of maps, atlases, and statistical and other reports offered for sale by the Federal Power Commission as authorized by section 825k of title 16 ; or (8) articles or documents to educational institutions or public libraries, or to Federal, State, or other public authorities.