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.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Geary - Geary is an email application built for GNOME 3.
K-9 Mail - K-9 is an opensource email client making it easy to chew through large volumes of email.