ProtonMail is an end-to-end encrypted email service based in Switzerland. Since its debut, it has become a viable email service for those wanting email security. Until now, you could only access your ProtonMail account via the web app, and an iOS app. But ProtonMail Bridge finally lets you bring it to the desktop. Bridge acts as a local email server using IMAP and SMTP. It runs in the background and interacts with email clients on your computer. All encryption and decryption happens locally. Benefits include full text search, multiple accounts, and import/export of emails. It uses JavaScript cryptography, and is open source. The software is only available for users with paid accounts, however. You can download it here.
Check It Out: ProtonMail Bridge Lets You Use Apple Mail as a Client