An Offline Address Book, or OAB, is an Address List used by Outlook clients running in Cached Exchange Mode for address book lookups when Outlook has no connectivity to Exchange. It reduces the workload on Exchange, as these clients will query their local OAB first. If an organization has all its clients running the Outlook Web App or Outlook in Online Exchange Mode, an OAB is not required.

Exchange 2007 and 2010 use one or multiple CASs and a web or Public Folder distribution method to distribute OABs, but the generation of an OAB is always bound to a single mailbox server. The obvious disadvantage here is that this server is a single point of failure and if it is unavailable for a long period of time, the generation of the OAB is affected and clients will not have an updated list of contacts.

Exchange 2013 changed the way OABs are generated and distributed. As for their generation, the OAB is now generated by the mailbox server that hosts a particular...