The basic assumption every privacy-concerned person should have about email is that it's never secure. Unless you use an offline cryptography program to encrypt your email text and then paste it into the email body before you send it, your emails are insecure.
Email was never designed with that in mind. If you want to communicate securely with somebody, use a medium/method that has been designed from the start for that purpose.
I use ProtonMail because it's not a massive corpo and it's open source, but I don't believe that my emails are significantly more secure than on a service like Exchange or Gmail.