The necessary bits to facilitate that could be added on top of the existing protocol in a manner that doesn't break existing clients. Essentially it amounts to an out of band registration of the expected sender with your own server, likely by means of a short proxy code or phrase. Couple with key exchange to facilitate an E2EE extension at the same time, while also dodging the logistical issue that would otherwise arise when a sender has multiple addresses or the sending address changes.

You can call it Secure-Email or RFC-99999

Yeah, because email as a family of protocols never developed different capabilities /s