Protonmail was used by Edward Snowden when leaking info about the US government and not only could the NSA not hack into it, but the company didn't turn over any information to the feds because they set-up their site to not capture any user data. That's about as solid a record as you could have.

All emails are encrypted (on your side at least, unless you're emailing someone else on protonmail) with a key that is larger in size than the data it is encrypting. This makes it mathematically impossible to separate the encrypted data from the key that's encrypting it. Just remember to use good OPSEC and don't make it easy to find your login credentials!