I'm trying to use the calendaring free/busy feature between 2 zimbra installations. When I attempt to query the free/busy status of a user, I get the below error:

2010-03-30 09:02:38,048 ERROR [btpool0-96://mail.ztest.com.com/service/soap/GetFreeBusyRequest] [name=jkimtest@ztest.com;aname=admin;mid=32;ip=x.x. x.x;ua=ZimbraWebClient - [unknown] (Linux)/6.0.5_GA_2213.RHEL5_64;] fb - can't find free/busy provider for user bryon@etest.com

Is there any additional configuration required to get this working?