10 C.F.R. § 1017.27 – Transmission.

(a) Physically transmitting UCNI documents or material.
1. (1) A document or material marked as containing UCNI may be transmitted by:
  1. (i) U.S. First Class, Express, Certified, or Registered mail;
  2. (ii) Any means approved for transmission of classified documents or material;
  3. (iii) An Authorized Individual or person granted limited access under § 1017.21 as long as physical control of the package is maintained; or,
  4. (iv) Internal mail services.
2. (2) The document or material must be packaged to conceal the presence of the UCNI from someone who is not authorized access. A single, opaque envelope or wrapping is sufficient for this purpose. The address of the recipient and the sender must be indicated on the outside of the envelope or wrapping along with the words “TO BE OPENED BY ADDRESSEE ONLY.”
(b) Transmitting UCNI documents over telecommunications circuits. Encryption algorithms that comply with all applicable Federal laws, regulations, and standards for the protection of unclassified controlled information must be used when transmitting UCNI over a telecommunications circuit (including the telephone, facsimile, radio, e-mail, Internet).