Le Cabinet Noir (literally, the Black Office) operated from the time of Louis XV and was one of the State's secret services responsible for censoring mail through systematic espionage. Once intercepted, letters were placed in a small red leather portfolio, the contents of which were concealed under the neutral moniker "Foreign Gazettes". *
* As a teenager I too had a small red leather portfolio in which I concealed my - ahem - "Foreign Gazettes".
The premises in which the secret service employees worked were connected to the office of the Post Office Director General who oversaw the Cabinet Noir. Letters that required examining were pushed through a hatch to specialized agents who promptly opened them, making a copy of those likely to be of interest to the king who, if he deemed necessary, would forward them to his ministers. The National Assembly attempted to curtail the work of the Cabinet Noir, but its services were too valuable. The office continued to function as a spy service up through Napoleonic times.