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.
Kiwi for Gmail - Dedicated Gmail client with clean integration of Google Calendar, Contacts, Google Drive, and...
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Mia for Gmail - A desktop email client for your Mac, without a browser !
Geary - Geary is an email application built for GNOME 3.
Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps