I can highly recommend MailMate. It’s not cheap for an email application, but it’s been rock-solid for me for the two or so years I’ve been using it. If you’re using Gmail with it, follow the instructions carefully at https://manual.mailmate-app.com/account_setup - Gmail’s fake IMAP implementation make configuring MailMate to work with it non-trivial.
What I like:
- Plain interface - means you get in and out of email as quickly as possible.
- Markdown editing - just like these forums!
- Automatable - I recently used their command line interface to automate sending a bunch of emails, which was amazing! See https://manual.mailmate-app.com/emate for documentation.
- Integration with other productivity apps, including OmniFocus, BusyCal, Fantastical, etc.
- Use of default
message://
links, rather than proprietary links likespark://
orairmail://
.