You'll more than likely need to upgrade all your Outlook clients to at lease 2007. We've seen issues with free/busy time not visble in Exchange 2010 with the older existing clients.
If I understand correctly, your Exchange CAL no longer includes the right to run Outlook, you must purchase it separately or as part of your Office suite. This may not be a big deal to some, but it does represent a major change in licensing.
I feel like Exchange is more trouble than it is worth. If you need to host an e-mail server Linux software does everything you need for free. E-mail does not need to be fancy, it needs to work. Why would you pay more money for a product that ends up being a time sink?
I would have to say, go with something with support. Sure, any Linux Distro you run has plenty of end user support. However, it's nothing like the support you get some a house like Microsoft. I may not be the biggest fan of the cost of their end user software, but you can't argue that they have come a LONG way in the past 10 years in terms of their server architecture.
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