MS exchange alternatives for FreeBSD

[Cheffo]

Hi list,

I have server running postfix + courier-imap, and I'm looking for 
someway to add possibility to exchange calendars/contacts/meeting 
invitations/etc between mail clients.

Can someone recommend calendar, that is compatible with windows clients 
and can be run under FreeBSD?

Thanks in advance.

P.S. Please CC me on reply as I'm not subscribed to -questions.

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Best Wishes,
Stefan Lambrev
ICQ# 24134177