Article 145-4-2 of the French Code of Criminal Procedure
When an accused person is remanded in custody, the examining magistrate may decide to prohibit him/her from corresponding in writing with one or more persons that he/she designates, in view of the requirements of the investigation, the maintenance of good order and security or the prevention of offences. For the same reasons, he may decide to withhold correspondence written by the detainee or addressed to him. The decisions referred to…