Kolab is the mail server system developed as the server component for the Kroupware project.
Kolab integrates Postfix and Cyrus IMAP with a powerful OpenLDAP database easily managed through the included PHP frontend. Because of its extremely powerful components, Kolab can be easily customised to suit the needs of any enterprise.
Kolab is a full groupware server similar to Microsoft Exchange, supporting Free/busy, shared calendaring, a communal LDAP address book for contacts, as well as fully recognising the iCal and vCard standards for interoperation with other servers.
A Kolab server is accessible from Kontact, the Kroupware project's reference client, as well as Aethera, the commercial client from theKompany.com. Third party plugins are available for other groupware clients like the evil Microsoft Outlook. Because the backend is IMAP it is of course possible to use any IMAP capable mail client.
A recent development in the Kolab project was the completion of the Horde webclient support for it. Horde is a powerful webmail system and the Kolab Horde supports full calendaring and groupware functionality, much like Microsoft Outlook Web Access for Exchange. This is an important step towards commercial recognition for Kolab.
When I first installed Kolab I was pleasantly surprised that it had configured full SSL/TLS support across the board for all of the components. This was something I wasnt expecting it to do out of the box so it was a definate bonus.
Because of its powerful components, Kolab is a definite option for any enterprise looking for a featureful, accessible, open source alternative to Exchange or Lotus Domino.