By default, Microsoft Outlook uses a cached mode configuration. Because cached mode generates an offline address book (OAB), understanding how to the offline address book works is important.
Cached mode is the default configuration for Outlook. When you are running in cached mode, your Exchange Online mailbox is synchronized to a local file (an .ost file), and the offline address list from Exchange Online is synchronized to a collection of files (.oab files) on your client computer. Outlook directly accesses the .ost file and the .oab files on your local computer instead of communicating directly with the servers. Therefore, network communication between Outlook and Office 365 is significantly reduced.
The offline address book contains a set of all the properties of a user (such as email addresses and names) that Outlook requires. With an offline address book, Outlook does not have to connect to the global catalog to resolve names, nor does it have to open a person’s detailed record. Instead, Outlook easily obtains this information from the local offline address book.
The offline address book is a snapshot of the Active Directory directory service information that is available in the global address list. Therefore, some information is available in the global address list that is not available in the offline address book. The offline address book does not contain the following items that are available in the global address list:
Outlook must be able to access the server to obtain this information. Therefore, if you are in cached mode and if you are working online (that is, your connection status indicates “Connected”), Outlook uses both the offline address book and the global address list to provide a complete listing of user information. If you are in cached mode and if your connection status shows either “Disconnected” or “Offline,” you can see only the information that is available in the offline address book.
If left constantly running, Outlook in cached mode automatically updates the offline address book on the client every 24 hours. The 24-hour time period is measured from the time that the offline address book was last downloaded successfully. For example, if you complete an offline address book download at 9:00 AM today, Outlook will start the offline address book download the next day at approximately 9:00 AM. Therefore, different people will receive updates at different, seemingly random times.
For a change that is made by AD Sync or made in CloudPortal Services Manager (CPSM) to reach the client computer, the following events must occur:
Unless you notice the last time that Outlook downloaded the offline address book, you may not know when Outlook is scheduled to try the next offline address book download. No indicator in the Outlook user interface advises you of the offline address book download schedule. Therefore, to manually force a download of the latest offline address book (assuming a new one is available), use one of the following methods:
Outlook compares the sequence ID of its last download with the sequence ID of the full/differential OAB file in Exchange Online. If Outlook determines that the sequence ID is higher than the one that was previously downloaded, Outlook triggers the download.
The offline address book files are located in the
%userprofile%\Local Settings\Application Data\Microsoft\Outlook folder. The folder contains six files for a full-details offline address book or five files for a no-details offline address book. The file names identify whether the offline address book is in Unicode format or in ANSI format. The following table lists the file names that are included.
|Details.oab||Udetails.oab||This file is not present in a no-details offline address book.|