Well, I have no experience doing what you're trying, but looks like a problem with your LDAP requests related to filter searchs. I suggest a deep log inspection or even a traffic capture to know what kind of communications are there between your mail clients and OpenLDAP.
Or just use 256 logging on the server side to see what type of searches Outlook is sending. Rememer of course that Outlook expects to be talking to an AD server, not an OpenLDAP server, and so it looks for a lot of AD specific stuff. You can usually modify it's configuration to get some very basic results back.
--Quanah -- Quanah Gibson-Mount Principal Software Engineer Zimbra, Inc -------------------- Zimbra :: the leader in open source messaging and collaboration