Protonmail advertises special end to end encryption thats only available when sending emails to other protonmail users. Others get the standard TLS that every provider uses. Recently they added a feature convert emails a link to a page that asks the non-protonmail user for a password (not a private key or login) and then decrypts the email in the browser, which I find pretty worthless.
Oh, and they dont support PGP with non protonmail users, locking you into their proprietary junk.
That's suboptimal but worthless might be too strong a word to describe it.
It's definitely better than nothing.
The proprietary thing does not look good I admit. But maybe they think the open stack is easy to pwn?