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An other Linux Small Business Server is Zentyal. It's based on Ubuntu server and plays very nicely with all packages you can get from Ubuntu. I like the LDAP master-slave mode to deploy multiple systems using centralized users&groups management, even you can sync with an existing Active Directory! Zentyal also has a Zarafa module (the perfect open source Microsoft Exchange replacement), RADIUS and a couple of this more I couldn't find on ClearOS. Give it a try!