You can't have SMTP if it's fully end-to-end encrypted.
That's why proton has a bridge, it decrypts messages locally, then runs an SMTP server for your local clients.
You can if the service doesn't handle encryption with a proprietary protocol and instead helps you set up PGP. It certainly has its issues and limitations, but after trying ProtonMail/Tutanota/what-have-you, I have _never_ actually used their E2E encryption except when contacting support for that mail service.
Meanwhile with PGP I can post up my public key on my personal sites / resume / social accounts, and people actually have reached out. I also like that you don't even need to include your email address to prevent scrapers from harvesting it. If it's on one of the public keyservers, their client will find the email address for the respective public key.
Well, ProtonMail isn't end-to-end encrypted anyway, even with Bridge. Unless you use PGP encryption. Emails are encrypted in-flight with TLS, but ProtonMail terminate that TLS connection.
I'm talking about zero knowledge encrypted email storage.
Proton mail doesn't have access to your messages, and SMTP doesn't support that type of encryption.
