Exchange 2016: Offline Address Books


An offline address book (OAB) is a replica of an address list collection which is been downloaded such that a Microsoft Outlook user is allowed to access the address book while disconnected from the server.


  • A new OAB files are generated by Microsoft Exchange and then compresses the files and then places them on a local share.

  • The address lists can be decided that is to be made available to the users who work offline, and you are also allowed to configure the method via which the address books are distributed.


Please Note:

The Microsoft Exchange OABGen service produces the OAB data, which is a mailbox assistant. In case you use the security descriptor to restrict users from accessing certain recipients in Active Directory, then the users who download the OAB will be eligible to view those hidden recipients. Hence, to hide a recipient from an address list, decide and set the HiddenFromAddressListsEnabled parameter on the Set-PublicFolder, Set-MailContact, Set-MailUser, Set-DynamicDistributionGroup, Set-Mailbox, and Set-DistributionGroup cmdlets. Subsequently, you are permitted to create a new default OAB that doesn’t contain the hidden recipients.


Moving OABs among Exchange versions

  • In Exchange 2007 and Exchange 2010, generally the Move-OfflineAddressBook cmdlet is used to move the OAB generation to another Mailbox server. Exchange 2013 supports only OAB (version 4).

  • This is the very same version that was the default in Exchange 2010. You are not allowed to configure Exchange 2013 to generate other OAB versions, and the OAB generation happens on the Mailbox server on which the company mailbox resides.

  • Hence, to move OAB generation in Exchange 2013, you should move the organization mailbox.

  • You are allowed only to move the OAB generation to another Exchange 2013 mailbox database. You are not allowed to move OAB generation to a previous version of Exchange.


To explore and find the Exchange 2013 OAB organization mailbox, run the following Shell command:


  • Get-Mailbox -Arbitration | where {$_.PersistedCapabilities -like “*oab*”}

  • Then you can use the MoveRequest cmdlets to shift the mailbox.


OAB version 4 and Outlook clients

  • Exchange 2013 only supports OAB version 4. And the OAB version 4 was introduced in Exchange 2003 Service Pack 2 (SP2) and is supported by Outlook 2007, Outlook 2010, and Outlook 2013.

  • This Unicode OAB permits client computers to get differential updates instead of full OAB downloads and a reduced file size.


Outlook clients that use OAB version 4

  • For Outlook 2013, Outlook 2010, Outlook 2007, and clients which use OAB version 4, in case the size of the changes.oab files is half the size (or more) of the complete OAB files, Outlook initiates a complete OAB download.


Web-based distribution

  • Web-based distribution is the distribution strategy in which Outlook 2013, Outlook 2010, or Outlook 2007 clients working offline can access the OAB.

There are various advantages to using Web-based distribution, inclusive of:

  • Support of more concurrent client computers.

  • Reduction in bandwidth usage.

  • Better control over the OAB distribution points. With Web-based distribution, the distribution point is the HTTPS web address where client computers permitted to download the OAB.

  • To benefit most from Web-based distribution, client computers should be running Outlook 2013, Outlook 2010, or Outlook 2007.

To function properly, Web-based distribution depends on the following components:

OAB generation process

  • It is the process via which Exchange creates and updates the OAB. For creating and updating the OAB, the OABGen service runs and executes on the Mailbox server on which the enterprise mailbox is placed. To support OAB distribution, this server should be an Exchange Mailbox server.


OAB distribution

  • In case a client initiates the OAB distribution request, the request is directed via a Client Access server. The Client Access server then routes and directs the request to the Mailbox server which is hosting the OAB files. Then, OAB files are distributed directly from the Mailbox server over to the client.


OAB virtual directory

  • The OAB virtual directory is those distribution point generally used by the Web-based distribution method.

  • Automatically, by default, when Exchange is setup and installed, a new virtual directory named OAB is created in the default internal website in Internet Information Services (IIS).

  • In case you have client-side users which connect to Outlook via outside your organization’s firewall, you are allowed to add an external website.

  • Subsequently, when you execute the New-OABVirtualDirectory cmdlet in the Shell, a new virtual directory with name OAB is created in the default IIS website present on the local Exchange Client Access server.


Autodiscover service

  • This is a function is available in Outlook 2013, Outlook 2010, Outlook 2007, and in few mobile devices which automatically configure the clients to have access to Exchange.

  • The service executes on a Client Access server and returns the correct OAB URL for a specific client connection.
    OAB main considerations

Irrespective of whether you use a single OAB or several OABs, consider the following points in your best practices for planning and implementing your OAB strategy:


  • Size of each OAB in your organization.

  • Number of OAB downloads.

  • Number and frequency of parent distinguished name changes.

  • SMTP addresses mismatches.

  • Overall number of alterations made to the directory.


OAB size considerations

  • For few organizations, the OAB is a small file which remote users occasionally download. For such organizations, downloading the OAB is generally not a concern.

  • But, for few large organizations which have large directories, or for enterprises which have deployed Outlook 2003 in Cached Exchange Mode, it may be a concern, mainly if the organizations have consolidated Exchange servers into a regional datacenter.


OAB sizes might vary from a few megabytes to a few hundred megabytes. The size of the OAB can be affected because of the following reasons:


  • Usage of certificates in an organization. The more the public key infrastructure (PKI) certificates, the bigger will be the OAB. And the PKI certificates range from 1 kilobyte (KB) to 3 KB.

  • They contribute to the single largest contributor to the OAB size.

  • Number of mail recipients located in Active Directory.

  • Number of distribution groups located in Active Directory.

  • Information which an organization adds to Active Directory for each and every mailbox-enabled or mail-enabled object.

  • For instance, few organizations populate and exploit the address properties on every user; others don’t.


Kristin is a content strategist at Techarex Networks. Kristin follows the B2B technology space closely and loves to write on the latest changes in technology, futuretech and fixes for day to day how to issues. Besides writing Kristin also loves music, moves and skating.

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