The OP mentioned Thunderbird but no-one else - shame, Thunderbird works very well and I have been using it for many, many years on both Linux and Macs. Very stable, very reliable, assorted add-ons (like browsers) and generally easy-to-use.
A friend of mine uses "Mutt" but really...
Postbox is heavily based on Thunderbird and is much nicer to use on a Mac.
It comes with a free 30-day trial period as well, so well worth a shot.
$40 to buy, but it is a lifetime license.