The best I have found is Thunderbird, which is pretty awful :(
It's been awhile since I last looked.
Spring Mail had a visual appeal but I didn't try it.
If you want something for free that works in Windows and Linux, you could try Claws Mail but it is very similar to Thunderbird.
If you are willing to pay for an email client, the best one is THE BAT!, it encrypts your emails on disk, supports PGP, has email filtering and a ton of other features. I tried many email clients over the years and nothing beats THE BAT!
https://protonmail.com/bridge/install
Evolution is a decent email program if you are on Linux, because if you are on Windows your secure email doesn't matter because your OS is it's own trojan horse.
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