Please note that, as most other people and organizations, I use a 3rd party Mail Service Provider. The staff who work at the provider are technically able to read all incoming & outgoing mail. This is how a email infrastructure works, and it’s irrelevant whether SSL or another encryption is used between email clients and corresponding email servers.
If your message is private, use PGP/GPG encryption to secure communications.
All email sent from my ProtonMail account should be signed with GPG.