Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues
Mailtrim - Email lives forever!. Reinvigorate your emailing experience with the best free email client, the Mailtrim
Newton Mail - Newton is a fully featured mail client available for Android, iOS, macOS and Windows.
Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.
Aqua Mail - Android Mail Client.
Missive - Team communication app now supporting FB, Twitter & SMS