As far as I can tell this also happens with the web client but I'll mention it here.
Pretty straightforward steps to reproduce:
1. Create an appointment.
2. If it isn't already open, open it and click on the schedule tab.
3. In the attendees list, enter an email address that is not on your Zimbra server.
Expected result: the freebusy line for that address should show in the color labeled as "unknown" in the key.
Actual result: the freebusy line shows in the color for "free". Also, a "?" appears to the right of the address; hovering the mouse over the "?" produces a popup that says "Status: Needs Action".