ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
Tutanota - Get your encrypted mailbox for free.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.