Some interesting numbers can be found in the research of the Radicati Group. This company specializes in researching e-mail, security, instant messaging and social networking with a focus on enterprise market.
In the study entitled “Microsoft Exchange Server and Outlook Market Analysis, 2010-2014”, we see that on-premises Exchange in 2010 has 76% of mailboxes as opposed to 15% being hosted and 9% from managed Exchange.
