Yep I checked it. I never compressed Outlook data files by fear of a slowdown.
But I added the registry entries found at How to configure the size limit for both (.pst) and (.ost) files in Outlook 2007 and in Outlook 2003. It should take care of this problem.
But still the suggestion by Rubble that a mode would allow the connector to " queries zimbra headers, fetches bodies on demand, etc? Fetch a month of data at a time" would be a big step forward.