Zimbra Mail Small

Zimbra is an enterprise-class email, calendar and collaboration solution, built for the cloud, both public and private. With a redesigned browser-based interface, Zimbra offers the most innovative messaging experience available today, connecting end users to the information and activity in their personal clouds. Zimbra is more or less a open-source replacement for your Microsoft Exchange solution. There is also a Desktop software (similar to Microsoft Outlook) available for most platform: Download Zimbra

Zimbra uses among many these open source software projects: