Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
Gammadyne Clyton - Clyton is an email client with advanced security and spam filtering features.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
mailcow - An open source mailserver suite.
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.