Inky makes exchanging encrypted and digitally signed email a breeze. It's as simple as regular email and works with any type of email account.
Visit - http://inky.com
Nylas N1: The open source, extensible email app Nylas N1. The extensible, open source mail app.
Nylas N1 developer version is free to use with an option to build application.
Visit - https://www.nylas.com/
eM Client is a full featured e-mail client witha modern and easy to use interface. eM Client alsooffers calendar, tasks, contacts and chat.
eMClient offers two versions:
2) Premium - USD 19.95 with No annual subscription! Lifetime license
Thunderbird is a free email application that’s easy to set up and customize - and it’s loaded with great features!
ThunderBird is an Opensource software developed by thousands of passionate, experienced developers and security experts spread all over the world.
Visit - https://www.mozilla.org/en-US/thunderbird/features/
Looking for an Outlook alternative? Download Opera Email client for free. It is easy to use, fast and customizable.
Opera Email also with new exciting features packed in beta version. Try it out.
Visit - http://www.opera.com/computer/mail