> but email doesn't even have the basic mechanisms HTTP has to prevent secrets from accidentally being sent in the clear.
There is a draft standard [1] for strict transport security. I'm not sure of its current status though.
> Email leaks metadata. In fact, some of what we call email "metadata" isn't even metadata --- stuff like subject lines are simply content. They're sent in plaintext.
Nothing sent during the data phase of the SMTP transaction has to strictly be unencrypted. There's nothing preventing the client from encrypting both the headers and body of the message. But that doesn't prevent the MTA itself from adding uncrypted headers to the message after the client sends the CRLF.CRLF sequence to end the data phase.
> Most email users get their email from a website. Unless you make them install something on all their computers
I'm not sure how many users still use a dedicated email client, but I'm sure that there are many who use clients like Outlook, Apple Mail, Thunderbird, amongst others. Anyone who wants to encrypt their email needs to know they need to use a client on their machine that handles the encryption/decryption steps for them (or that they need to do it themselves.
[1] https://www.infoworld.com/article/3046850/security/google-mi...