Postbox is a pretty great Thunderbird derivative. <a href="https:&#x2F;&#x2F;www.postbox-inc.com&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.postbox-inc.com&#x2F;</a><p>I love Airmail, but require something cross-platform. Postbox isn&#x27;t as pretty, but is cross platform, priced right, and has great default config out of the box.