19 C.F.R. § 145.3 : Customs Duties — U.S. CUSTOMS AND BORDER PROTECTION, DEPARTMENT OF HOMELAND SECURITY; DEPARTMENT OF THE TREASURY — MAIL IMPORTATIONS — GENERAL PROVISIONS — Opening of letter class mail; reading of correspondence prohibited

(a) Matter in addition to correspondence. Except as provided in paragraph (e), Customs officers and employees may open and examine sealed letter class mail subject to Customs examination which appears to contain matter in addition to, or other than, correspondence, provided they have reasonable cause to suspect the presence of merchandise or contraband.
(b) Only correspondence. No Customs officer or employee shall open sealed letter class mail which appears to contain only correspondence unless prior to the opening:
(1) A search warrant authorizing that action has been obtained from an appropriate judge of United States magistrate, or
(2) The sender or the addressee has given written authorization for the opening.
(c) Reading of correspondence. No Customs officer or employee shall read, or authorize or allow any other person to read, any correspondence contained in any letter class mail, whether or not sealed, unless prior to the reading:
(1) A search warrant authorizing that action has been obtained from an appropriate judge or United States magistrate, or
(2) The sender or the addressee has given written authorization for the reading.
(d) Other types of correspondence. The provisions of paragraph (c) shall also apply to correspondence between school children and correspondence of the blind which are authorized to be mailed at other than the letter rate of postage in international mail.
(e) Certain Virgin Islands mail. First class mail originating in the Customs territory of the United States and arriving in the U.S. Virgin Islands, which is to be delivered within the U.S. Virgin Islands, shall not be opened unless:
(1) A search warrant authorizing that action has been obtained from an appropriate judge or United States magistrate, or
(2) The sender or the addressee has been given written authorization for the opening.

19 C.F.R. §145.3

T.D. 78-102, 43 FR 14454, 4/6/1978