MS didn't do it (unless they managed to bribe all the developers). That's the only platform that software that met the specs we needed was available on. And I'm still waiting on the open source Exchange Killer app.
It's possible, but I've also seen a lot of specs written to get one specific application, not to the actual needs.
And I'm still waiting on the open source Exchange Killer app.
OS X Server does have enterprise-capable mail and calendaring functions integrated with the directory, and they're based on open standards so, unlike with Exchange, you're not locked-in. It's included for unlimited clients, as opposed to Microsoft, which charges tens of thousands of dollars to get unlimited clients from Exchange -- per server, of course.