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.
Evolution - Apps/Evolution - GNOME Wiki!
Roundcube - Web-based IMAP email client
Postbox - The Power Email App for busy professionals, like you.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.