19 C.F.R. § 145.2 : Customs Duties — U.S. CUSTOMS AND BORDER PROTECTION, DEPARTMENT OF HOMELAND SECURITY; DEPARTMENT OF THE TREASURY — MAIL IMPORTATIONS — GENERAL PROVISIONS — Mail subject to Customs examination

(a) Restrictions. Customs examination of mail as provided in paragraph (b) of this section is subject to the restrictions and safeguards relating to the opening of letter class mail set forth in §145.3.
(b) Generally. All mail arriving from outside the Customs territory of the United States which is to be delivered within the Customs territory of the United States and all mail arriving from outside the U.S. Virgin Islands which is to be delivered within the U.S. Virgin Islands, is subject to Customs examination, except:
(1) Mail known or believed to contain only official documents addressed to officials of the U.S. Government;
(2) Mail addressed to Ambassadors and Ministers (Chiefs of Diplomatic Missions) of foreign countries; and
(3) Letter class mail known or believed to contain only correspondence or documents addressed to diplomatic missions, consular posts, or the officers thereof, or to international organizations designated by the President as public international organizations pursuant to the International Organizations Act (see §148.87(b) of this chapter). Mail, other than letter class mail, addressed to the designated international organizations is subject to Customs examination except where the organization certifies under its official seal that the mail contains no dutiable or prohibited articles. Any Customs examination made shall, upon request of the addressee international organization, take place in the presence of an appropriate representative of that organization.

19 C.F.R. §145.2

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