https://www.techdirt.com/articles/20210908/17064547528/protonmail-turned-over-french-activists-ip-address-to-law-enforcement-following-request-swiss-authorities.shtml

ProtonMail has long advertised itself as a particularly privacy-conscious email service. The free end-to-end encrypted email service promises more privacy and security than many of its competitors. But there are limits. ProtonMail operates out of Switzerland, making it subject to that country's laws (which, to be fair, are hardly draconian). It also (at least temporarily) retains a certain amount of information about users' emails -- metadata that can be used to verify accounts in the case of a lost password.

And while email between ProtonMail accounts is encrypted, the same protection isn't applied to emails between services, like communications sent to or from ProtonMail from other email services. This is an understandable limitation, which is why many seeking secure communications have moved to encrypted messaging services, rather than email offerings that collect metadata about communications.