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I spent the last days of my summer vacation trying to find a GPL'd Exchange replacement. After considering all aspects I fell down on Kolab, so I was surprised to see it escaping your attention. It is still early, they just got mobile syncing in place this summer, but they do address all aspects you crossed in this article. Rather than attempting reverse-engineering a moving target, they have built a completely open solution that should scale very nicely across a wide range of company sizes, simply distributing the load with master/slave configurations. Everything you need is in one easy-to-install package, so administrators actually have an easier time administrating it than Exchange or Domino, e.g., ClamAV and SpamAssasin is included and works out-of-the-box.