Correspondence of the members of the diplomatic corps of the countries of the Postal Union of the Americas and Spain stationed in the United States may be reciprocally transmitted in the domestic mails free of postage, and be entitled to free registration without right to indemnity in case of loss. The same privilege is accorded consuls and vice consuls when they are discharging the function of consuls of countries stationed in the United States, for official correspondence among themselves, and with the Government of the United States.
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39 U.S. Code § 3217 - Correspondence of members of diplomatic corps and consuls of countries of Postal Union of Americas and Spain
Statutory Notes and Related Subsidiaries
Free Mailing Privileges Continue Unchanged
“Nothing in this Act [see Tables for classification] or any amendment made by this Act shall affect any free mailing privileges accorded under section 3217 or sections 3403 through 3406 of title 39, United States Code.”