Finally, open source\ufffds biggest problem in many enterprises isn\ufffdt technical at all; it\ufffds business-oriented: Many companies will be leery about building a critical resource such as e-mail on a platform with no support-contract safety net. Things will work fine as long as your Linux administrator is happy, but should he or she quit, the company will be left without an immediate recourse.
IOW make sure your employees all know they're replaceable. It's better to be at the mercy of Microsoft, than the guy you just screwed out of his weekend with your poor planning.