Mutt. The learning curve is substantial and the omnipresence of HTML and graphics is email will make the experience poor… but if you take part in multiple mailing lists and are 99% text and little else, it’s worth the hassle.
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It doesn't need to have all the "AI" that Superhuman has but keyboard shortcuts and hotkeys are a must. Must also have a mobile app
I've tried Spark, Shortwave, Apple Mail, Gmail, Thunderbird and none of them really compare.