GPGMail - GPGMail is an open source plugin for Apple Mail.
Roundcube - Web-based IMAP email client
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.