Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Alpine - Alpine is a fast, easy to use email client that is suitable for both the inexperienced email user...
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Tutanota - Get your encrypted mailbox for free.