Epyon

Nope, it’s agnostic of email provider; you just need a public/private key pair and an email client that can encrypt and decrypt emails, such as Thunderbird.

How it works, is that both you and the person you’re communicating with each create public/private keys using a tool such as GnuPG. You both publish your public keys to https://keys.openpgp.org/. You never share your private key; if you accidentally reveal or share it, you’ll need to revoke the public key and generate a new pair. When you’re ready to send an email, you download your friend’s public key, and then you encrypt the email with that public key and send it off. The email is encrypted in both transit and at rest on both your email provider’s server and your friend’s server. Your friend then opens the email and uses their private key to decrypt and read it. When they’re ready to reply, they do the same thing with your public key and you can decrypt and read it with your private key.

This guide by the Free Software Foundation can get you up and going! Feel free to shoot me a DM if you want to try sending/receiving encrypted email.

Tuta is probably the closest direct replacement.

I left Proton though for Fastmail. Ultimately I just needed a Gmail replacement and it ticked all of my boxes. If I need to encrypt email I’ll just do it myself with OpenPGP.