Polymail - Native email app for email productivity.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Airmail for iOS - One of the best Mac email clients, now on iPhone
Roundcube - Web-based IMAP email client
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.