Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
Missive - Team communication app now supporting FB, Twitter & SMS
Mia for Gmail - A desktop email client for your Mac, without a browser !
Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.
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.
Mailtrim - Email lives forever!. Reinvigorate your emailing experience with the best free email client, the Mailtrim