MailMate - MailMate is an alternative email client that was designed for the macOS.
The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Geary - Geary is an email application built for GNOME 3.
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