MS Exchange Alternatives?

Slashdot has an interesting discussion as a reader asks: Slashdot | Can We Finally Ditch Exchange?: “With all the innovations going into open source software these days, why do I still need to run Exchange to meet my clients’ needs? Even when demonstrating technology like LTSP mixed with any combination of OpenOffice, Star Office, even Codeweaver’s Crossover Office running the latest Microsoft suite, the clincher is always over Exchange functionality. I’m aware of Bynari’s InsightServer(Coincidentally, I noticed on that page, that their code is for sale) and have started using that as a possible closer, and the cost is much less prohibitive than eXchange w2k server CALs; but why isn’t there an open source solution to this problem yet?”