We have a custom software that has built in calendaring. It can dump those entries into an Exchange server calendar. I tried inputting the Zimbra server and of course that didn't work. I contacted the Vendor and they will only support Exchange, but are working on gmail sync as well. They have no ETA when that will be released.
The only solution I can think of off the top of my head is to run an Exchange server for calendaring only. Have the software sync with Exchange, then have Exchange sync with Zimbra. Of course I'd rather not have to go the Exchange route, so can anyone think of something else?
I did create a ticket with Zimbra support last week, but haven't heard back.