So when it comes to encryption for digital data there are really two concerns:
- encrypted at rest
- encrypted in transit
Your options for encrypted email providers are limited:
- https://www.privacyguides.org/en/email/
- https://en.wikipedia.org/wiki/Comparison_of_webmail_providers#Features
and Protonmail seems incorrect for this because it largely wouldn't be encrypted mail.
I'm curious why you think so?
Proton's inboxes are encrypted, so that's (1) handled.
For (2), Proton-to-Proton emails are automatically encrypted. Proton-to-WKD-enabled-services are also automatically encrypted:
Proton also supports automatic external key discovery with Web Key Directory (WKD). This means that emails sent to other providers which use WKD will be automatically encrypted with OpenPGP as well, without the need to manually exchange public PGP keys with your contacts.
And finally, emails to non-secure services can be encrypted, but you must provide the decryption password to the receiver through some other method. These emails can also be configured to automatically delete after a set expiration time.
This is the most feature-complete encrypted email service that I'm aware of, it basically covers all cases that it is possible for Proton to cover on their own service, anything more would require cooperation from the other service(s). No email service could possibly force an inbound email to be encrypted in transit, the sending service has to do that, and that's really the only part that Proton doesn't have a feature for (because it's impossible). If encryption is your concern, I don't think there are any better options right now.