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Convention (XI) relative to certain Restrictions with regard to the Exercise of the Right of Capture in Naval War. The Hague, 18 October 1907.
Chapter I : Postal correspondence - Art. 2.
Art. 2. The inviolability of postal correspondence does not exempt a neutral mail ship from the laws and customs of maritime war as to neutral merchant ships in general. The ship, however, may not be searched except when absolutely necessary, and then only with as much consideration and expedition as possible.