Another issue with encryption is the other aspect of security; malware detection. Since E2E encryption happens at the MUA level, and most spam/phishing/malware

Another issue with encryption is the other aspect of security; malware detection. Since E2E encryption happens at the MUA level, and most spam/phishing/malware scanning is done either at the MTA level or by a gateway, detection becomes almost impossible. MUA would need to do all the extra heavily lifting. Email was designed as a plain-text messaging system. PGP, MIME and S/MIME are extensions to this, but fundamentally, email is still plain text. The key to secure email is not to use email for secure things.