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Good call. I think the differences b/w the two would be notable. PostPath is said to be a "drop-in alternative to MS Exchange" and even works alongside existing Exchange servers in the same environment. Also, full collaboration features, Outlook compatibility, etc. without the use of plugins. I'm unfamiliar with Open-Xchange, so I don't know how it stacks up in that regard. Another big difference is that Open-Xchange is GPL, whereas PostPath is "free for up to 12 users."

Looks like TR blogged about it just recently:
http://blogs.techrepublic.com.com/networking/?p=174