Wikipedia
Zentyal
Zentyal is an open source email and groupware solution, natively compatible with Microsoft Outlook. Zentyal implements Microsoft Exchange Server protocols on top of standard open source components (such as Dovecot, Postfix, Samba, etc.) in order to provide native compatibility with Microsoft Outlook clients. The email and groupware protocols supported by Zentyal are MAPI, ActiveSync, EWS, SMTP, POP, IMAP, CalDAV, CardDAV and Active Directory.
Zentyal is distributed in two packages: Zentyal Server for SMBs and Zentyal Cloud for hosting providers. Zentyal Server has a development edition that is freely downloadable and its source code is available under terms of the GNU General Public License.