Rumours of its death are greatly exaggerated: https://www.mozilla.org/en-GB/thunderbird/

I steer well clear of EOL software

In reply to a post by Chiny:

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.

I also use Airmail 3 which I think is well worth the £7,99

In reply to a post by 23Prince:

I also use Airmail 3 which I think is well worth the £7,99

Agreed. Another good option, although personally not a great fan of their screen presentation particularly with the way it displays the spam folder (It's either too small at the bottom of the screen or doesn't display an unread message count if you move it the main folders in favorites.)

Much better option than the Mac Mail app which doesn't play very nicely with the way that Gmail manages spam though.

Yup. I found it to be even worse when it came to Exchange. Gmail works fine