Outlook: Sharing calendars between one Exchange account and one Zimbra account

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Hi,

I have received a little "special" request from a customer. She is on our hosted Zimbra system. Her husband uses Outlook with an Exchange server at work. They would like to have a shared "family" calendar, but the husband doesn't want to manage two separate calendars in two different systems and would like the calendar to sync with his Blackberry. However, they're OK with getting a Zimbra account for him if that solves the problem.

So, my question is: If we set him up with a Zimbra account and he configures two accounts in Outlook, one on his corporate Exchange server (already exists) and one for our hosted Zimbra using ZCO, is there anyway his two Exchange calendars can be synced with his Zimbra calendar?

I know there used to be lots of little sync conduits, like Palm's Desktop sync app out there before, which essentially, from a technical perspective, does what I'm after ... just with another OS and PIM app. Maybe the solution to my "problem" is really simple and I'm just overlooking it?

Any input and ideas would be greatly appreciated. Thanks in advance.