32 C.F.R. § 2001.46 – Transmission.

(a) General. Classified information shall be transmitted and received in an authorized manner which ensures that evidence of tampering can be detected, that inadvertent access can be precluded, and that provides a method which assures timely delivery to the intended recipient. Persons transmitting classified information are responsible for ensuring that intended recipients are authorized persons with the capability to store classified information in accordance with this Directive.
(b) Dispatch. Agency heads shall establish procedures which ensure that:
1. (1) All classified information physically transmitted outside facilities shall be enclosed in two layers, both of which provide reasonable evidence of tampering and which conceal the contents. The inner enclosure shall clearly identify the address of both the sender and the intended recipient, the highest classification level of the contents, and any appropriate warning notices. The outer enclosure shall be the same except that no markings to indicate that the contents are classified shall be visible. Intended recipients shall be identified by name only as part of an attention line. The following exceptions apply:
  1. (i) If the classified information is an internal component of a packable item of equipment, the outside shell or body may be considered as the inner enclosure provided it does not reveal classified information;
  2. (ii) If the classified information is an inaccessible internal component of a bulky item of equipment, the outside or body of the item may be considered to be a sufficient enclosure provided observation of it does not reveal classified information;
  3. (iii) If the classified information is an item of equipment that is not reasonably packable and the shell or body is classified, it shall be concealed with an opaque enclosure that will hide all classified features;
  4. (iv) Specialized shipping containers, including closed cargo transporters or diplomatic pouch, may be considered the outer enclosure when used; and
  5. (v) When classified information is hand-carried outside a facility, a locked briefcase may serve as the outer enclosure.
2. (2) Couriers and authorized persons designated to hand-carry classified information shall ensure that the information remains under their constant and continuous protection and that direct point-to-point delivery is made. As an exception, agency heads may approve, as a substitute for a courier on direct flights, the use of specialized shipping containers that are of sufficient construction to provide evidence of forced entry, are secured with a combination padlock meeting Federal Specification FF-P-110, are equipped with an electronic seal that would provide evidence of surreptitious entry and are handled by the carrier in a manner to ensure that the container is protected until its delivery is completed.
(c) Transmission methods within and between the U.S., Puerto Rico, or a U.S. possession or trust territory—(1) Top Secret. Top Secret information shall be transmitted by direct contact between authorized persons; the Defense Courier Service or an authorized government agency courier service; a designated courier or escort with Top Secret clearance; electronic means over approved communications systems. Under no circumstances will Top Secret information be transmitted via the U.S. Postal Service or any other cleared or uncleared commercial carrier.
(2) Secret. Secret information shall be transmitted by:
1. (i) Any of the methods established for Top Secret; U.S. Postal Service Express Mail and U.S. Postal Service Registered Mail, as long as the Waiver of Signature block on the U.S. Postal Service Express Mail Label shall not be completed; and cleared commercial carriers or cleared commercial messenger services. The use of street-side mail collection boxes is strictly prohibited; and
2. (ii) Agency heads may, when a requirement exists for overnight delivery within the U.S. and its Territories, authorize the use of the current holder of the GSA contract for overnight delivery of information for the Executive Branch as long as applicable postal regulations (39 CFR. Chapter I) are met. Any such delivery service shall be U.S. owned and operated, provide automated in-transit tracking of the classified information, and ensure package integrity during transit. The contract shall require cooperation with government inquiries in the event of a loss, theft, or possible unauthorized disclosure of classified information. The sender is responsible for ensuring that an authorized person will be available to receive the delivery and verification of the correct mailing address. The package may be addressed to the recipient by name. The release signature block on the receipt label shall not be executed under any circumstances. The use of external (street side) collection boxes is prohibited. Classified Communications Security Information, NATO, and foreign government information shall not be transmitted in this manner.
(3) Confidential. Confidential information shall be transmitted by any of the methods established for Secret information or U.S. Postal Service Certified Mail. In addition, when the recipient is a U.S. Government facility, the Confidential information may be transmitted via U.S. First Class Mail. However, Confidential information shall not be transmitted to government contractor facilities via first class mail. When first class mail is used, the envelope or outer wrapper shall be marked to indicate that the information is not to be forwarded, but is to be returned to sender. The use of streetside mail collection boxes is prohibited.
(d) Transmission methods to a U.S. Government facility located outside the U.S. The transmission of classified information to a U.S. Government facility located outside the 50 states, the District of Columbia, the Commonwealth of Puerto Rico, or a U.S. possession or trust territory, shall be by methods specified above for Top Secret information or by the Department of State Courier Service. U.S. Registered Mail through Military Postal Service facilities may be used to transmit Secret and Confidential information provided that the information does not at any time pass out of U.S. citizen control nor pass through a foreign postal system.
(e) Transmission of U.S. classified information to foreign governments. Such transmission shall take place between designated government representatives using the government-to-government transmission methods described in paragraph (d) of this section or through channels agreed to by the National Security Authorities of the two governments. When classified information is transferred to a foreign government or its representative a signed receipt is required.
(f) Receipt of classified information. Agency heads shall establish procedures which ensure that classified information is received in a manner which precludes unauthorized access, provides for inspection of all classified information received for evidence of tampering and confirmation of contents, and ensures timely acknowledgment of the receipt of Top Secret and Secret information by an authorized recipient. As noted in paragraph (e) of this section, a receipt acknowledgment of all classified material transmitted to a foreign government or its representative is required.