Not having mandatory security is a legit issue, but there isn’t a drop-in replacement that does, not in 2024. You’re gonna need widespread support, support for file transfer, federated operation, resistance to abuse, client software on many platforms, etc.
And email security is way down the list of things that I’d be concerned about. At least with email, you’ve got PGP-based security. If you’re worried about other people’s mail providers attacking mail you send them, that’s getting into “do you trust certificate authorities to grant certificates” territory, because most secure protocols are dependent upon trusting that.
Like, XMPP with OTR is maybe a real option for messaging, but that’s not email.
EDIT: Not to mention that XMPP doesn’t mandate security either.
Estebiu