Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Cypht - All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.