My organization standardizes on Mozilla Thunderbird as our mail client of choice (partly because Mail.app is so deficient on OSX). It’s a bit crusty since Mozilla halted further development in 2012, but better the devil you know, right…?
This is a companion discussion topic for the original entry at https://www.funkypenguin.co.nz/how-to/addons-to-improve-thunderbird-and-make-it-less-annoying/