Code of Federal Regulations Title 43. Public Lands:  Interior § 43.20.509 Penalty mail and official stationery

(a) An employee is prohibited from using any official envelope, label, or indorsement authorized by law, to avoid the payment of postage or registry fee on his or her private letter, packet, package, or other matter in the mail (18 U.S.C. 1719).

(b) Official Government envelopes and official letterhead stationery are Government property that may only be used for authorized purposes. Employees' use of Government envelopes to mail their own personal job applications is not authorized.