Exchange; that is quite a huge one for businesses where I am. Many people are just so used to outlook for email that when it comes for them to put in some kind of email management architecture they will always go exchange.
Actually I was thinking more in terms of web-server or any application server that is not tied to the OS. I am guessing you can't host Exchange server on Linux even if you wanted to. So that advantage would't really be an advantage. Its more like you don't have any option but to use Windows server.
There are some non-Windows Exchange alternatives that would be transparent to Outlook users, but it's probably an uphill battle selling them to management.
Yes. You'll have better chance selling a new paradigm (cloud based, no clients to deploy, no individual mail server mattering) than a opened version of the old paradigm (which is still controlled by the proprietary vendor).