I am running Mac OS X Leopard Server on my Mac Pro with multiple clients, all using Mac OS X Leopard. Each client also has Microsoft Office 2008 for Mac installed with Entourage. Is it possible to install Microsoft Exchange Server on the Mac Pro and use it to set up calendaring for all of my Mac clients through Entourage?
Apologies for Jeff's rather useless answer.
You can absolutely install Microsoft Exchange on a Mac Pro server to accompany OS X Server.
Head over to VMware, Parallels or Oracle and grab a virtualisation hypervisor. I personally recommend VMware ESXi.
Setup a Virtual Machine running a copy of Microsoft Windows Server 2012 and Microsoft Exchange Server 2013. (More information here http://www.microsoft.com/en-us/server-cloud/products/windows-server-2012-r2/buy.aspx#fbid=f4pyYTvvgji and here http://office.microsoft.com/en-us/exchange/microsoft-exchange-server-2013-business-email-email-server-try-now-FX103765014.aspx)
If you would rather not involve yourself with all the fuss involved with maintaining Microsoft Exchange, head over to Microsoft Office's website and Subscribe to Office 365 for Business. If you would rather not work with Microsoft at all, head over to Google and setup Google Apps.