At last! I have been searching for something like eM Client for many months and only now have I found it. I have an e-mail/calendar/contacts environment based on OS X (Mountain Lion) Server supporting a mixed set of Windows and Mac clients. eM Client is the *only* Windows client that I have found (and I have tried many including of course Thunderbird) that combines a robust IMAP implementation with fully functional CalDAV and CardDAV capabilities. Fabulous!
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with best regards