While using PowerShell to setup free/busy requests on Exchange, you receive the following message: GetUserAvailability Error : Microsoft.Exchange.InfoWorker.Common.Availability.AddressSpaceNotFoundException: Configuration information for forest/domain xxx.com could not be found in Active Directory

You must add an availability  address space for your domain to instruct Office 365 to forward free/busy requests.

Please see http://kb.cloudiway.com/how-to-force-addresslist-gal-in-office-365/ for step-by-step instructions.