Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
MailTab for Gmail - MailTab for Gmail is the ideal app for instantly accessing your Gmail account without having to open your web browser.
Mia for Gmail - A desktop email client for your Mac, without a browser !
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
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.
Wavebox - The powerful productivity browser like no other, multi-account sign-in, tab sleeping, groups & pins