Zimbra is open source server software for email and collaboration - email, group calendar, contacts, instant messaging, file storage and web document management. The Zimbra email and calendar server is available for Linux, Mac OS X and virtualization platforms. Zimbra syncs to smartphones (iPhone, BlackBerry) and desktop clients like Outlook and Thunderbird. Zimbra also features archiving and discovery for compliance. Zimbra can be deployed on-premise or as a hosted email solution.
Yahoo! Zimbra Desktop is a free email and calendar client which runs on any Windows, Apple or Linux desktop computer. It works online and offline and works with any POP or IMAP email account as well as Yahoo! Mail.