Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
Morning Mail - A new way to manage your mail. Tinder for email.
Mia for Gmail - A desktop email client for your Mac, without a browser !
Polymail - Native email app for email productivity.
Mailplane - Mailplane is the best way to use Gmail on your Mac. Use an unlimited number of accounts, get new mail notifications, use plug-ins and much much more.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.