Besides criminalizing the use of encrypted instant messengers, French prosecutors also incriminate the use of services such as Protonmail which ensures email confidentiality, the use of tools to protect one’s privacy on the internet (VPN, Tor, Tails), the use of techniques to protect against GAFAM surveillance, the encryption of digital media or even the organization of training sessions on digital protection (crypto-parties).

You just have to be doing something illegal if you don’t want other people reading your email.