ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
HEY - Email at its best, new from Basecamp.
Tutanota - Get your encrypted mailbox for free.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.