HEY - Email at its best, new from Basecamp.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
InstAddr - Disposable email address application.
GPGMail - GPGMail is an open source plugin for Apple Mail.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
ElectronMail - Unofficial desktop app for ProtonMail and Tutanota end-to-end encrypted email providers