Polymail - Native email app for email productivity.
Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.
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
Keybase Chat - End-to-end encrypted chat built into Keybase
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Crypho - End-to-end-encrypted messaging and file sharing