I am using Zimbra 6 and invited an external person using Apple iCal 4.0.1. The email with the reply containing an ics file was obviously not recognized by Zimbra, because the appointment status for the external person did not change.
Replies from Outlook users invited in the same way are automatically recognized by Zimbra and the status in the calender is updated. So the process in itself obviously works. There seems to be a problem with the format of the ics file, which differs a lot between Apple iCal and Outlook.
Is this a bug?