[rompledorph]

Are there any schemes that could allow e2ee seamlessly across email providers without compromise in privacy?

[dvzk]

In theory, S/MIME and SMIMEA. In reality, normal users demand account data recovery in cases of lost encryption keys and passwords. So key escrow is required, which isn't E2EE. ProtonMail tells forgetful users to shove it, which limits its adoption; meanwhile ProtonMail also supports webmail (another feature requirement) which decrypts messages in the browser, weakening any E2EE claims.