the average user doesn’t control their mail server

fair point. there are options however. you are not locked into trusting a specific entity. but the critical point is that even signal is able to figure out who is talking to whom: https://sanesecurityguy.com/articles/signal-knows-who-youre-... sure, for SMTP the contact details are directly in the messages, which is worse, but i don't know of any service that works completely without metadata. but signal is at least trying.

also strict transport security for this property. This is not widely true of mail servers on the Internet

since gmail requires TLS i highly doubt that there are many servers out there that don't support it.

the average user has multiple high-quality E2EE messaging technologies available to them

available and willing to switch are different. as i said, my friends are not willing to sign up to yet another messaging service. it's a social media fatigue.

why people think Signal shares your phone number with people other than recipients

that's not the point, at least for me. i am hesitant share my number with signal or any other service, and worse, i do not want to share my number with the people i talk to. i refused to use signal until the later was fixed. i refused whatsapp too, but to many people that i need to reach demand it, so i had no choice.

these are all trade-offs. not everyone agrees on the same, and while i understand and principally agree with your arguments, for me they don't work because i can't convince my friends. i also have other friends who do run their own mail servers. i have contacts who require whatsapp and others who can only use wechat. most often i don't have a choice. i am using whatever i can get people to agree to, and for that deltachat is a good option. signal could have been a better option but unfortunately their requirement to share phone numbers until recently made them a worse option than deltachat or even telegram for anything but 1:1 communication with trusted friends (those who i trusted to have my number). that has changed now, and i started to use it. but it will take time to build up my contacts there. btw, in some countries it is not even possible to sign up to signal. the number gets rejected.

> since gmail requires TLS i highly doubt that there are many servers out there that don't support it.

Gmail doesn’t require TLS, unless by that you mean that their webmail interface is TLS only. Like every other mail provider, they do opportunistic TLS on external delivery, and TLS on MUA connections (SMTP and IMAP) is largely at the mercy of user configuration.

The fact that people seem to think that TLS is a mainstay of the email ecosystem is clearly part of the problem here.

As for the rest of this: I’ve hammered on about Signal because it’s the naive right choice, but it’s ultimately up to you to decide whether your phone number is an acceptable public identifier. But even if it isn’t, there is so much out there that’s indisputably better than this mess: Matrix or even iMessage (with an email identifier instead of a phone) would be better.