It is forbidden to the public to send by mail:
1st. Letters or packets containing gold or silver substances, piece of money, jewelry, or precious articles;
2d. Any packets whatever containing articles liable to customs duty.
In case a packet falling under one of these prohibitions’ is delivered by one Administration of the Union to another Administration of the Union, the latter proceeds according to the manner and forms prescribed by its legislation or by its interior regulations.
There is, moreover, reserved to the Government of every country of the Union, the right to refuse to convey over its territory, or to deliver, as well articles liable to the reduced rate, in regard to which the laws, ordinances, or decrees which regulate the conditions of their publication or of their circulation in that country have not been complied with, as correspondence of every kind which evidently bears inscriptions forbidden by the legal enactments or regulations in force in the same country.