Zimbra Collaboration Suite Is a Email Server for Organizations

Zimbra Collaboration Suite (ZCS) is a  Mailing solutions core product for any level of enterprise.It’s an email and calendar server plus much more; think about it like a next-generation Microsoft Exchange server. In addition to email and calendar, it provides document storage and editing, instant messaging, and simplified administrative controls all in an award winning webmail user interface built with the latest AJAX web technology. ZCS also provides mobility and syncs to desktop client applications; the server is deployed on commodity Linux and Mac server hardware.

There are several versions of ZCS to choose from:

ZCS Open Source Edition (free)

ZCS Network Edition