Wavebox - The powerful productivity browser like no other, multi-account sign-in, tab sleeping, groups & pins
Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
Mia for Gmail - A desktop email client for your Mac, without a browser !
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.
Unibox - Unibox is an application that aims to completely overhaul your email experience. Unibox resorts your email so that you can group the messages you receive by their sender. Read more about Unibox.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.