I do not want to go off-topic, but that is wrong. The majority of "regular" email systems do not use encryption in the backend and all messages are still stored in cleartext form on the servers. Trust me, I was a sysadmin for years. 
I was away yesterday but I was reading your response and Googled some articles about email security for different email services. I had a wrong idea that Google do PGP encryption but knowing they don't, just broke my trust. You are right most regular email system don't. Protonmail has it as an additional feature, which needs additional setting to enable it in your end. And if the receiver do not have the same setting then they will not be able to read your email. It become complicated when the email is sent to another service instead of proton to proton.