I've recently migrated from Exchange with great success. I don't know wether a webbased client is acceptable in your case, but I have to say
Group-Office is quite a brilliant solution.
It's GPL-ed, allthough an officially supported version is also available. It has all the functionality you mentioned
Quote:
shared and private agenda; shared and private contacts, private mail, todo list
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You can use any IMAP/POP3 solution as an e-mail backend. I use Postfix w/ MySQL and Courier IMAP.