ProtonMail published a nice blog post explaining how encrypted email works, and the various protocols that companies use.
End-to-end encryption for messages sent between ProtonMail users is automatic, and our integrated OpenPGP support makes it easy to send and receive PGP-encrypted E2EE messages to people that use PGP with other email providers. Proton also informs you when your messages are protected by E2EE with a small blue padlock (for other ProtonMail users) or green padlock (for OpenPGP users).
Check It Out: Everything You Wanted to Know About How Encrypted Email Works