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
HEY - Email at its best, new from Basecamp.
Mia for Gmail - A desktop email client for your Mac, without a browser !
Canary Mail - Privacy-aware email app for Mac & iOS
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.