Here are some resources to help you: Zimbra 7 versus Exchange 2010 (PDF) Embrace and Extend: Making Open Technologies Displace Incumbents in the Enterprise by Kevin Henrikson (good overview of Zimbra) Zimbra vs Microsoft Exchange a feature and cost comparison Zimbra versus Microsoft Exchange 2008 Zimbra versus Microsoft Exchange 2007 ZDNet Australia, Top Alternatives to Microsoft Outlook CNET Apparently Microsoft prefers Zimbra, too CNET Microsoft's curious infatuation with Zimbra, redux We have also collated various Zimbra presentations and these are available on slideshare.net/agileware There is also a great collection of case studies and webinars available at http://www.zimbra.com/about/webinars.html Agileware can also provide customer references, trials and consulting to help you with your business case.