ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Tutanota - Get your encrypted mailbox for free.
Mail Backup X - Your one stop mail backup and archiving tool for Mac.
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
Mail Archiver X - Mail Archiver X protects one of your Mac-based small business' most important assets.