@beardalaxy You're right. You can't 100% prove if a service is 100% reliable. You need to make an educated guess based on the terms of service, privacy policy, the history of the service, etc. I wouldn't recommend ProtonMail for the shear fact that you can't use third-party clients like Thunderbird which by proxy means you can't generate your own GPG keys if you want to send encrypted email. And ProtonMail has openly admitted to giving data to various governments. It's probably better than Gmail, but not ideal. I wouldn't recommend it if you were doing something like black/gray market trading.

Personally, I would recommend either cock.li (if you can get an invite), fedora.email (which is run by @sjw) or Posteo (which costs 1 EUR per month). However, if you have the technical knowledge, a spare computer, and a decent enough internet speed, hosting your own email server is the best option.

However, email is a fundamentally flawed system. It's probably the most insecure electronic communication system that is widely used today. XMPP+OMEMO encryption is probably your best bet if you want to have the most secure means of communication.