There's no MUA-STS, there's no good way to enforce TLS for email clients. Most allow errors to be trivially bypassable. MTA-STS between MTA's does help, but IM

There's no MUA-STS, there's no good way to enforce TLS for email clients. Most allow errors to be trivially bypassable. MTA-STS between MTA's does help, but IMHO MTA-MTA is not the biggest issue.