Secrecy of correspondence
The secrecy of correspondence (German: Briefgeheimnis, French: secret de la correspondance) or literally translated as secrecy of letters, is a fundamental legal principle enshrined in the constitutions of several European countries. It guarantees that the content of sealed letters is never revealed, and that letters in transit are not opened by government officials, or any other third party.
Source: Wikipedia — Secrecy of correspondence (CC BY-SA 4.0)