39 U.S.C. § 3202: Penalty mail

officers of the Government of the United States other than Members of Congress; the Smithsonian Institution; the Pan American Union; the Pan American Sanitary Bureau; and the United States Employment Service and the system of employment offices operated by it in conformity with the provisions of sections 49–49c, 49d, 49e–49k of title 29, and all State employment systems which receive funds appropriated under authority of those sections. mail relating to naturalization to be sent to the Immigration and Naturalization Service by clerks of courts addressed to the Department of Justice or the Immigration and Naturalization Service, or any official thereof; and mail relating to a collection of statistics, survey, or census authorized by title 13 and addressed to the Department of Commerce or a bureau or agency thereof. A department or officer authorized to use penalty covers may enclose them with return address to any person from or through whom official information is desired. The penalty cover may be used only to transmit the official information and endorsements relating thereto. This section does not apply to officers who receive a fixed allowance as compensation for their services including expenses of postage.