Polymail - Native email app for email productivity.
Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
Canary Mail - Privacy-aware email app for Mac & iOS
Mia for Gmail - A desktop email client for your Mac, without a browser !
Flow-e - Turn your Gmail or Office365 inbox to a Visual Task Board.
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.