The offline address book contains the properties of a user, such as email addresses, that Microsoft Outlook requires to send an email message and display information about the sender. By using an offline address book, Outlook doesn’t have to connect to Microsoft Exchange Server to resolve names or open the records for each user. This reduces network traffic and improves performance.

Note:  A Microsoft Exchange Server account is required.

The offline address book is a snapshot of information from the Global Address List (GAL). Therefore, not all the information in the GAL is available in the offline address book. The following information in the GAL might not be available in the offline address book:

  • Custom properties that are added by a server administrator, such as the employee ID of each employee.

  • Organizational hierarchy.

  • Group memberships.

Outlook must be able to access Exchange to obtain this information. Therefore, if you are working in Cached Exchange Mode and if you are working online — that is, your connection status is Connected to Microsoft Exchange — Outlook uses both the offline address book and the GAL to provide a complete listing of user information. If you are in Cached Exchange Mode and your connection status is Disconnected, Trying to Connect, or Offline, you can see only the information that is in the offline address book.

Change offline address book download details

You can choose how much detail to download by doing the following:

  1. Select the File tab.

  2. Select Account Settings.

  3. Select Download Address Book or Download changes since last Send/receive.

  4. If you see an option for Information to download, select Full Details or No Details.

Note: If you're using Cached Exchange Mode, we recommend that you download the full details of the offline address book. This reduces the number of times the Outlook window might become unresponsive and the number of unnecessary connections to the Microsoft Windows Server Active Directory.

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More information about offline address book updates

If Outlook is left running constantly in Cached Exchange Mode, it automatically updates the offline address book about one time per day depending on updates on the computer that is running Exchange.

The offline address book size can vary from several megabytes to hundreds of megabytes in uncompressed form. The following factors can affect the size of the offline address book:

  • The usage of security certificates in your organization. They are the single largest contributor to the size of the offline address book.

  • The number of users in Windows Server Active Directory Domain Services.

  • The number of distribution groups in Windows Server Active Directory Domain Services.

  • The information that your organization has added to Windows Server Active Directory Domain Services for each user and each distribution group. For example, some organizations populate the address properties for each user.

Outlook has the following safeguards to help protect your organization's network from too many offline address book downloads:

  • In the default configuration, only one full offline address book download attempt is made in a 13-hour period. Each time when a server response is received and the download starts is counted as a download attempt.

  • In the default configuration, Outlook doesn’t limit the number of attempts for differential updates. If a differential update fails, another attempt is made after one hour.

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