The oab.xml file requires authenticated users granted permission to read for the OAB to download properly from Outlook clients. I've noticed when the OAB gets copied from \ExchangeOAB to \ClientAccess\OAB via FDS the permissions get reset and this breaks OAB download. Authenticated users is no longer there after permissions reset. I've changed NTFS to not inherit permissions and it still occurs. I'm not certain from where and why the permissions are getting overwritten from. Any ideas appreciated.