Forgot one:
Mailpilot (previously discussed e.g., here and here) announced MailPilot 5 sometime in the past year (edit: year or so) but I don’t think there’s been any updates since. The developer, Alexander Obenauer, is still doing stuff, so maybe someone here has tried it! It looks slick, and it promises to be cross-platform.
Edit: added Mail Pilot 5 to the original post.