As posted elsewhere in another comment, https://protonmail.com/blog/protonmail-threat-model/

Not recommended

If you are attempting to leak state secrets (as was the case of Edward Snowden) or going up against a powerful state adversary, email may not be the most secure medium for communications. The Internet is generally not anonymous, and if you are breaking Swiss law, a law-abiding company such as ProtonMail can be legally compelled to log your IP address. A powerful state adversary will also be better positioned to launch one of the attacks described above against you, which may negate the privacy protection provided by ProtonMail. While we can offer more protection and security, we cannot guarantee your safety against a powerful adversary.