39 U.S.C. § 3217

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.