You gotta check this out!
www.zimbra.com
Zimbra Collaboration Suite (ZCS) 4.5 is a truly modern, innovative messaging and collaboration application. Zimbra is the leading open source solution for enterprise, service provider, education, and government environments; offering administrators and their end-users unmatched benefits. It's no wonder Zimbra is a popular choice for today's growing Mac and Linux email server base.
Ajax based web collaboration is at the heart of ZCS 4.5. The powerful web client integrates email, contacts, shared calendar, VoIP, and online document authoring into a rich browser-based interface. Also, our unique open source Zimlet technology makes it easy for you to include custom 'mash-ups' in the ZCS web client.
ZCS 4.5 also includes an elegant Ajax based Admin Interface plus full scripting tools to manage the ZCS server. Full support is provided for standards-based APIs (IMAP / POP / iCal / CalDAV) as well as MAPI and iSync, which enable seamless compatibility with other clients like Microsoft Outlook, Apple desktop suite, and Mozilla Thunderbird.
Zimbra also offers Zimbra Mobile, which provides over-the-air "push" synchronization to mobile devices without the need for third party software or additional servers for most devices.
The Zimbra solution also has a complete high security package including built-in anti-spam and anti-virus scanning.
Now Zimbra also features Archiving and Discovery, an optional component to save and search email for various compliance issues.