I work for a company that presenly uses POP3 Mail. There are various companies within the group. Each company has a registered domain on the internet so email is easy to setup and manage, All companies are located in the same building. Put simply, all users log in to the same local domain (i.e. group.local) for file sharing but their Outlook client uses POP3 and is confiured to coleect mail for their respective companies i.e. companyy1.com, compay2.com. The total number of users across all 4 organisations is 30. Some organisations have only 2 users, one 18 etc.
Now in order for to share contacts and calendars, we are thinking about going the exchange route. We are considering a hosted option. We have looked at MS365 but this is not an option at present as we would have to make a considerable investment in new hardware and Office 2010 licenses.