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.
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
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Newton Mail - Newton is a fully featured mail client available for Android, iOS, macOS and Windows.
Geary - Geary is an email application built for GNOME 3.
myMail - myMail is the smart email app for managing all your accounts on iOS and Android.