The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.
Mailbox for Zoho - Multi-Account Zoho client for Mac.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Geary - Geary is an email application built for GNOME 3.
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.