Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Mia for Gmail - A desktop email client for your Mac, without a browser !
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
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.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.