It is an e-mail and news cross-platform client software package by Mozilla Foundation. Thunderbird can manage multiple e-mail, newsgroup and RSS accounts and supports multiple identities within accounts. Features like quick search, saved search folders , advanced message filtering, message grouping, and labels help manage and find messages. Just like Firefox, the tons of extensions and themes for this client makes it very secure and flexible to to enhance your productivity.Download Mozila ThunderbirdZimbra Collaboration Suite (Open Source Version)
Zimbra is a client and server platform for messaging and collaboration. The web client integrates email, contacts, shared calendar, VoIP, and online document authoring in a rich browser-based interface. This is more like MS-Exchange and Outlook combo. In other words it is compatible with proprietary clients such as Microsoft Outlook and Apple Mail, both through proprietary connectors, as well as the open-source Novell Evolution, so that mail, contacts, and calendar items can be synchronised from these to the ZCS server. Zimbra also provides native two-way sync to many mobile devices such as Nokia Eseries, BlackBerry, Windows Mobile, iPhone with 2.0 softwareDownload Zimbra Collaboration Suite (Open Source Version)Sylpheed
Sylpheed is a free, GTK+-based, open source email and news client. It is very light lightweight. Sylpheed runs on Windows, Mac OS X and Unix-like systems such as Linux, and BSD.