Zimbra Collaboration Server is a great alternative to Microsoft Exchange. At Frontware we moved from Exchange to ZCS in order to integrate more Open Source tools, improve email performance and integration with Alfresco and OpenERP. We installed Zimbra on a Ubuntu server, our email system is now 100% based on Open Source products.
Zimbra Collaboration Server is the most capable open source email and collaboration suite available today. Far outstripping what is possible with simple open source mail servers, Zimbra provides organizations major productivity boosts with global address lists, shared calendars and document management on the web or offline with Zimbra Desktop. Support for standard protocols enables use of virtually any desktop client or device for added end user flexibility.
Installation and administration is straightforward; all the interdependent components required by Zimbra are packaged within the server, eliminating any integration work by the administrator and management of the system is easily performed using the rich AJAX Admin Console. A vibrant community of IT experts, who are capable of handling most technical questions, supports Zimbra Collaboration Server Open Source Edition.