Email can be made secure, it's just that by default it's insecure and there isn't a Google/Mozilla/Microsoft/Apple conglomerate to penalize SMTP servers by showing "Insecure" in an email display/etc.

* Most mail transport agents can require connections to use TLS, but nobody does because they're afraid of denying incoming messages and because it can be difficult to install a certificate for an MTA, so smaller providers just don't. There should be a LetsEncrypt for mail servers and by default the various package managers and distributions should not install a mail agent that allows non-TLS connections.

* It is possible to store mail messages encrypted at rest, but only by using an encrypted filesystem-- I'm not aware of any MTA that will do this itself. Obviously that needs to change.

* Providers can force TLS connections for IMAP. This is surprisingly easier these days given that most end user mail clients will auto-negotiate these connections.

There's your E2E encryption. This doesn't get you encryption on the final destination, but that's a separate issue.