law: Article 27
  1. The receiving State shall permit and protect free communication on the part of the mission for all official purposes. In communicating with the Government and the other missions and consulates of the sending State, wherever situated, the mission may employ all appropriate means, including diplomatic couriers and messages in code or cipher. However, the mission may install and use a wireless transmitter only with the consent of the receiving State.
  2. The official correspondence of the mission shall be inviolable. Official correspondence means all correspondence relating to the mission and its functions.
  3. The diplomatic bag shall not be opened or detained.
  4. The packages constituting the diplomatic bag must bear visible external marks of their character and may contain only diplomatic documents or articles intended for official use.
  5. The diplomatic courier, who shall be provided with an official document indicating his status and the number of packages constituting the diplomatic bag, shall be protected by the receiving State in the performance of his functions. He shall enjoy personal inviolability and shall not be liable to any form of arrest or detention.
  6. The sending State or the mission may designate diplomatic couriers ad hoc. In such cases the provisions of paragraph 5 of this article shall also apply, except that the immunities therein mentioned shall cease to apply when such a courier has delivered to the consignee the diplomatic bag in his charge.
  7. A diplomatic bag may be entrusted to the captain of a commercial aircraft scheduled to land at an authorized port of entry. He shall be provided with an official document indicating the number of packages constituting the bag but he shall not be considered to be a diplomatic courier. The mission may send one of its members to take possession of the diplomatic bag directly and freely from the captain of the aircraft.