Roundcube - Web-based IMAP email client
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
HEY - Email at its best, new from Basecamp.
GPGMail - GPGMail is an open source plugin for Apple Mail.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
ElectronMail - Unofficial desktop app for ProtonMail and Tutanota end-to-end encrypted email providers