The Organization Mailbox is a new type of arbitration mailbox introduced with Exchange 2013.
Exchange 2013 supports only the web distribution method, so let’s explore the changes in web-distribution method.
The Exchange 2007/2010 CAS pulled the OAB files generated on the respective Mailbox server and stored them locally. CAS 2013 proxies all OAB download requests to the appropriate Exchange 2013 Mailbox server.
OABs are also critical in reducing the workload on Exchange servers as cached mode Outlook clients will always query the local OAB first. The last major overhaul of OAB architecture was in Exchange Server 2007, where we introduced web-distribution of OAB along with CAS server role taking major responsibility of distributing the OAB.
If this server was unavailable for a long period, the OAB generation was affected.
In Exchange 2013, the OAB is generated by each Exchange 2013 Mailbox server(s) that hosts a special type of arbitration mailbox, called parameter anymore.
With this change in the architecture, the Microsoft Exchange File Distribution Service is removed from the CAS role.
In Exchange 2013, this is the flow of OAB download: This new workflow overcomes the disadvantages of legacy OAB download workflow.
When you install the first Exchange mailbox server, setup designates it as the OAB generation server. When creating a new OAB, the OAB generation server has to be specified.OAB in Exchange Server 2010: Get-Offline Address Book “Default Offline Address Book” | fl name,server Name : Default Offline Address Book Server : MBX1 The disadvantage with this approach was that only one server was configured for OAB generation, and it was a single point of failure.