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Manual of the Laws of Naval War. Oxford, 9 August 1913.
Section IV : On the rights and duties of the belligerent with regard to enemy property - Art. 53.
Art. 53. B. Postal correspondence. Postal correspondence, whatever its official or private character may be, found on the high seas on board an enemy ship, is inviolable, unless it is destined for or proceeding from a blockaded port.
The inviolability of postal correspondence does not exempt mail-boats from the laws and customs of maritime war as to 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.
If the ship on which the mail is sent be seized, the correspondence is forwarded by the captor with the least possible delay.