Maybe Brendan Eich will put together an email client to compete with Thunderbird, like he did with “Brave” to compete with “Firefox” when Mozilla kicked him out for not being a leftist idiot.
Funny, just yesterday I was researching another email client to use on Linux.
I use Claws-Mail on Linux (and my wife on Windows). It has a conventional style GUI and is aimed at plain text, but it also can do spelling checking, plus display images, simple HTML, PDFs and more (some with plug-ins).
It doesn’t allow directly composing HTML messages, but you can attach any kind of file you wish. It has lots of other features such as message filtering, configurable toolbars and hotkeys, etc. It allows anti-spam filtering (with add-ons), Perl and Python scripting and can support encrypted email (e.g., using GPG)
One of it’s main advantages from my point of view is that it doesn’t try to be your secretary and organize (i.e., dictate) the way you work. (Even Thunderbird had a bit of that.)
We use Claws-Mail as an IMAP client, so all our mail is kept only on our local Dovecot IMAP server for privacy, but still can be accessed from more than one computer (even remotely, if an encrypted tunnel were used).
Claw-Mail can run on BSD, Linux, macOS, Solaris, Unix & Windows. And (of course) it’s free and Open Source.
1. https://www.claws-mail.org/
2. https://en.wikipedia.org/wiki/Claws_Mail
3. https://9to5linux.com/claws-mail-4-1-adds-text-zooming-in-the-message-view-many-other-new-features