I just learned of this one:

https://tutanota.com/blog/posts/desktop-clients/

 Secure desktop clients for Linux, Windows and Mac OS (Beta).

 Tutanota is an encrypted email service, available as web client
 with open source apps for Android and iOS. We invite you all to
 test our brand-new open source desktop clients with built-in
 encryption.

 2018-12-21

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The article is a bit dated, but has anyone here tried tutanota?


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