Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
Dragon Responsive Email Designer - Dragon Email Designer provides an intuitive drag and drop newsletter design tool.
Mia for Gmail - A desktop email client for your Mac, without a browser !
Marble Responsive Email Designer - Marble helps you design beautiful and mobile-friendly emails and newsletters.
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.
Kingmailer - Kingmailer helps you send, receive and route/forward emails.