Mail within United States Postal Channels
Department of Defense Regulation 5240.1-R, Procedures Governing the Activities of DOD Intelligence Components that Affect U.S. Persons, § C8.2.1 (Dec. 1982): “Mail within United States Postal Channels”
Mail Within United States Postal Channels includes:
C18.104.22.168. Mail while in transit within, among, and between the United States, its territories and possessions (including mail of foreign origin that is passed by a foreign postal administration, to the United States Postal Service for forwarding to a foreign postal administration under a postal treaty or convention, and mail temporarily in the hands of the United States Customs Service or the Department of Agriculture), Army-Air Force (APO) and Navy (FPO) post offices, and mail for delivery to the United Nations, NY; and
C22.214.171.124. International mail enroute to an addressee in the United States or its possessions after passage to United States Postal Service from a foreign postal administration or enroute to an addressee abroad before passage to a foreign postal administration. As a rule, mail shall be considered in such postal channels until the moment it is delivered manually in the United States to the specific addressee named on the envelope, or his authorized agent.