I ran into a rather obscure Exchange Availability Service behavior that will be of little interest to most. So, if you are not working at a hosting company or have never heard of the ‘msExchQueryBaseDN’ attribute, save yourself some time and skip this post.
The ‘msExchQueryBaseDN’ attribute is used to restrict Outlook Web Access’ (OWA) search for mail enabled objects in Active Directory (when simulating the Global Address List) — or at least that is what it was originally used for. Rather than searching for all mail enabled objects, it will search only a portion of Active Directory. The attribute is usually not set because most Exchange organizations have only one Global Address List – The ‘Default Global Address List’ which contains all mail enabled objects.
If you don’t understand why there would be more than one GAL, stop reading here.