Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
POP Peeper - POP Peeper is an email notifier that runs in your Windows task bar and alerts you when you have new...
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.
Wavebox - The powerful productivity browser like no other, multi-account sign-in, tab sleeping, groups & pins