Take a look at Google Apps as well. http://www.google.com/a. It is free and it allows you to use your own domain (not google.com). It is shared calendaring and contacts. It is a bit of an island right now relative to the other Google tools like docs, spreadsheets, groups etc. These things will likely talk to eachother eventually but not yet.
Yahoo also has a business email hosting.
I have heard solve360 is good.
And, I would def check out Zimbra. Georgeous AJAX interface with a fantastic tool set and good integration possibilities. Last I looked they were selling software and not service but they have since signed up some hosting partners.
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>>;I operate a small non-profit with three employees. Each employee's personal computer operates with XP and MS Outlook. I would like to enable the feature of the MS Outlook calendar that allows each employee to see what appointments have been made for the other. What software do we need to purchase in order to enable that feature of the MS Outlook calendar.>>
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