No joy getting Exchange users to see any free/busy info about Zimbra users. I'm beginning to think it's a lost cause -- not ready to completely give up, but this is definitely switching to the back burner after a quick survey of our users says it's not a huge issue if they can't see free/busy information for others for a while. Logs show this error:
At this point my conclusion is quite simply that it just doesn't work with Exchange 2013, that just enough was changed since 2010 that Zimbra can read it, but not push to it. If correct, then short of a Zimbra upgrade (which, at least for now, is not on the table) there's just no way to make this work.
2014-04-24 22:24:42,072 ERROR [EXCHANGE2010 Free/Busy Sync Queue]  fb - error commucating to https://exchange-1.example.com/EWS/Exchange.asmx