Plan a Cached Exchange Mode deployment in Outlook 2007

Updated: April 9, 2009

Applies To: Office Resource Kit

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Topic Last Modified: 2016-11-14

When Microsoft Office Outlook 2007 is configured for Cached Exchange Mode, the user can enjoy a better online and offline experience because a copy of the user's mailbox is stored on the local computer.

When an Office Outlook 2007 account is configured to use Cached Exchange Mode, Office Outlook 2007 works from a local copy of a user's Exchange mailbox stored in an Offline Folder file (OST file) on the user's computer, along with the Offline Address Book (OAB). The cached mailbox and OAB are updated periodically from the Exchange server.

Consider the following when you plan an Office Outlook 2007 deployment:

The primary benefits of using Cached Exchange Mode are the following:

  • Shielding the user from network and server connection issues

  • Facilitating switching from online to offline for mobile users

By caching the user's mailbox and the OAB locally, Outlook no longer depends on on-going network connectivity for access to user information. In addition, users' mailboxes are kept current. If a user disconnects from the network—for example, by removing a laptop from a docking station—the latest information is automatically available offline.

In addition to using local copies of mailboxes to improve the user experience, Cached Exchange Mode optimizes the type and amount of data sent over a connection with the server. For example, if On Slow Connections Download Headers Only is configured, Outlook changes the type and amount of data sent over the connection.

Outlook checks the network adapter speed on the user's computer to determine a user's connection speed, as supplied by the operating system. Reported network adapter speeds of 128 KB or lower are defined as slow connections. There might be circumstances when the network adapter speed does not accurately reflect data throughput for users. For more information about adjusting the behavior of Outlook in these scenarios, see the section Managing Outlook behavior for perceived slow connections later in this topic.

Outlook can adapt to changing connection environments by offering different levels of optimization, such as disconnecting from a corporate local area network (LAN), going offline, and then reestablishing a connection to the server via a slower dial-up connection. As your Exchange server connection type changes—for example, to LAN, wireless, cellular, or offline—transitions are seamless and never require changing settings or restarting Outlook.

For example, users might have a laptop computer at work with a network cable connection to a corporate LAN. In this scenario, users have access to headers and full items, including attachments. Users also have quick access and updates to the computer running Exchange Server. If users disconnect their laptops from the LAN, Outlook switches to Trying to connect mode. Users can continue to work uninterrupted with their data in Outlook. If they have wireless access, Outlook can reestablish a connection to the server and then switch back to Connected mode.

If the users later connect to the Exchange server by using dial-up access, Outlook recognizes that the connection is slow and automatically optimizes for that connection by downloading only headers and by not updating the Offline Address Book. In addition, Office Outlook 2007 includes optimizations to reduce the amount of data sent over the connection. Users do not need to change settings or restart Outlook during this scenario.

In addition to the Outlook 2003 Trying to connect and Connected modes, a new mode, Need Password, is introduced in Office Outlook 2007. The mode displays when Outlook is in a disconnected state but is not offline. This can happen, for example, when a user clicks Cancel in a credentials authentication dialog box. When Outlook is disconnected but is not offline, a user-initiated action (such as clicking Send/Receive) causes Outlook to prompt again for the password and to display Connected mode, even though Outlook is disconnected and is waiting for a password.

Some Outlook features reduce the effectiveness of Cached Exchange Mode because they require network access or bypass Cached Exchange Mode functionality. The primary benefit of using Cached Exchange Mode is that the user is shielded from network and server connection issues. Features that rely on network access can cause delays in Outlook responsiveness that users would not otherwise experience when they use Cached Exchange Mode.

Some Outlook features can require network access to retrieve information, such as looking up free/busy information. This can cause a delayed response, even when users have fast connections to Exchange data. The delays can occur unpredictably, rather than only when the feature is accessed by the user.

In addition, the following features might rely on network access and can cause delays in Outlook unless users have fast connections to Exchange data:

  • Delegate access, when folders are not cached locally (local cache is the default)

  • Opening another user's calendar or folder that are not cached locally (local cache is the default)

  • Using a public folder that is not cached

See Managing Outlook folder sharing in Synchronization, disk space, and performance considerations later in this topic.

In Office Outlook 2007, shared folders that users access in other mailboxes are downloaded and cached in the user's local OST file when Cached Exchange Mode is enabled. Only shared Mail folders are not cached. For example, if a co-worker shares a calendar with a user and the user opens it, Office Outlook 2007 starts caching the folder locally so that the user has offline access to the folder and is insulated from network issues. However, if a manager delegates access to his or her Inbox to a team member, accessing the folder is an online task and can cause response delays.

We recommend that you disable or do not implement the following features, or combination of features, if you deploy Cached Exchange Mode:

  • The toast alert feature with digital signatures on e-mail messages   Outlook must check a network server to verify a digital signature. By default, Outlook displays a toast message that contains a portion of an e-mail message when new messages arrive in a user's Inbox. If the user clicks the toast message to open a signed e-mail message, Outlook uses network access to check for a valid signature on the message.

  • Multiple Address Book containers   The Address Book typically contains the Global Address List (GAL) and user Contacts folders. Some organizations configure subsets of the GAL, which display in the Address Book. These subset address books can also be included in the list that defines the search order for address books. If subset address books are included in the search order list, Outlook might need to access the network to check these address books each time a name is resolved in an e-mail message that a user is composing.

  • Custom properties on the General tab in Properties dialog box for users   The Properties dialog box appears when you double-click a user name (for example, on the To line of an e-mail message). This dialog box can be configured to include custom properties unique to an organization, such as a user's cost center. If you add properties to this dialog box, however, we recommend that you not add them to the General tab. Outlook must make a remote procedure call (RPC) to the server to retrieve custom properties. Because the General tab shows by default when the Properties dialog box is accessed, an RPC would be performed each time the user accessed the Properties dialog box. As a result, a user running Outlook in Cached Exchange Mode might experience noticeable delays when he or she accesses this dialog box. To help avoid such delays, you should create a new tab on the Properties dialog box for custom properties, or include custom properties on the Phone/Notes tab.

Installing certain Outlook add-ins can affect Cached Exchange Mode. Some add-ins can access Outlook data by using the object model to bypass the expected functionality of Headers Mode (Download Headers Only) in Cached Exchange Mode. For example, full Outlook items—not just headers—download if you use Microsoft ActiveSync technology to synchronize a hand-held computer, even over a slow connection. In addition, the update process is slower than if you download the items in Outlook, because one-off applications use a less-efficient type of synchronization.

There are a number of issues to consider when you deploy Cached Exchange Mode. The way Cached Exchange Mode works to maintain a current local copy of a user's Exchange mailbox and other information can affect other Outlook features and behavior. In some cases, you can improve how Cached Exchange Mode works with other Outlook features for your whole organization or for a group of users (for example, users who work remotely).

Cached Exchange Mode works independently of existing Outlook Send/Receive actions to synchronize users' OST and OAB files with Exchange Server data. Send/Receive settings update users' Outlook data in the same way they did in earlier versions of Outlook.

Users who synchronize Outlook data by pressing F9 or clicking Send/Receive might not realize that manual synchronization is no longer necessary. In fact, network traffic and server usage can be affected if users repeatedly execute Send/Receive requests to Exchange Server. To minimize the effects, inform users that manual Send/Receive actions are unnecessary in Cached Exchange Mode. This might be especially helpful for remote users who typically used Outlook in offline mode with earlier Outlook versions and used Send/Receive to synchronize their data or just before they disconnected from the network. This type of data synchronization now occurs automatically with Cached Exchange Mode.

Another way to manage the issue is to disable the Send/Receive option for users. However, sometimes this can create problems for users, such as when you upgrade current Outlook users with POP accounts and existing customized Send/Receive groups to Office Outlook 2007. In this situation, disabling the Send/Receive option means users cannot download POP e-mail messages.

Cached Exchange Mode allows Outlook to access the local Offline Address Book (OAB) for user information, instead of requesting the data from Exchange Server. Local access to user data greatly reduces the need for Outlook to make remote procedure calls (RPCs) to Exchange, and shields the user from much of the network access required in Exchange online mode or in previous versions of Outlook.

After users have a current OAB installed on their computers, only incremental updates to the OAB are needed to help protect against unnecessary server calls. Outlook in Cached Exchange Mode synchronizes the user's OAB with updates from the Exchange Server copy of the OAB every 24 hours. You can help control how often users download OAB updates by limiting how often you update the Exchange Server copy of the OAB. If there is no new data to synchronize when Outlook checks, the user's OAB is not updated.

Although users with a No Details OAB can use Outlook with Cached Exchange Mode, we recommend that you install a Full Details OAB on users' computers. We also recommend that users use the Unicode OAB. The ANSI OAB files do not include some properties that are in the Unicode OAB files. Outlook must make server calls to retrieve required user properties that are not available in the local OAB, which can result in significant network access time when users do not have a Full Details OAB in Unicode format.

When you deploy Cached Exchange Mode for Outlook, be aware that users' OST files can increase 50 to 80 percent over the size of the mailbox reported in Exchange Server. The format Outlook uses to store data locally for Cached Exchange Mode is less efficient than the server data file format. This results in the use of more disk space when mailboxes are downloaded to provide a local copy for Cached Exchange Mode.

When Cached Exchange Mode first creates a local copy of a user's mailbox, the user's current OST file, if one exists, is updated. When users have relatively small mailboxes—for example, less than 500 megabytes (MB) of Exchange Server data—this works fine. However, ensure that users with larger mailboxes have Unicode-formatted OST files before you deploy Cached Exchange Mode. Unicode is an Outlook file format that was first provided in Outlook 2003. Unicode OST files can store up to 20 gigabytes (GB) of data, instead of the limit of 2 GB on non-Unicode (ANSI) Outlook files.

By creating Unicode OST files, you can help to avoid error messages for users. Error messages result when Outlook runs out of OST file space when it attempts to create a local copy of the user's mailbox for Cached Exchange Mode. Outlook with Cached Exchange Mode also works better when there is plenty of free space in the user's OST file; for example, when only 5 to 10 percent of a 20 GB OST file is used.

Also be sure that users' OST files are located in a folder with sufficient disk space to accommodate users' mailboxes. For example, if users' hard drives are partitioned to use a smaller drive for system programs (the system drive is the default location for the folder that contains the OST file), specify a folder on another drive with more disk space as the location of users' OST files. For more information about deploying OST files in a location other than the default location, see "To configure a default OST location by using Group Policy" in Configure Cached Exchange Mode in Outlook 2007.

Many factors influence a user's perception of Cached Exchange Mode performance, including hard disk size, CPU speed, and the expected level of performance. For example, offline users might find that Cached Exchange Mode provides better performance, while users who formerly accessed Exchange in online mode might perceive reduced Outlook performance.

One factor that can contribute to reduced performance is a large OST file. If the user's OST file grows too large (for example, larger than 1 GB), Outlook with Cached Exchange Mode performance degrades. To improve response time in Outlook, users should either reduce the size of their mailbox (for example, by archiving older files) or disable Cached Exchange Mode. To help prevent large OST files, you can set a limit on the mailbox size in Exchange Server. You might also choose to disable synchronizing shared non-mail folders or disable synchronizing users' Public Folder Favorites if you previously enabled the option in your deployment of Cached Exchange Mode.

In Office Outlook 2007, shared folders that users access in other mailboxes are downloaded and cached in the user's local OST file when Cached Exchange Mode is enabled. Only shared Mail folders are not cached. For example, if a co-worker shares a calendar with another user and the user opens it, Office Outlook 2007 starts caching the folder locally so that the user has offline access to the folder and is insulated from network issues. However, if a manager delegates access to his or her Inbox to a team member, accessing the folder is an online task and can cause response delays.

You can configure this option in the Office Customization Tool (OCT) when you customize your Cached Exchange Mode deployment. If users in your organization typically open many shared calendars and people work together on networks with reliable links to their Exchange servers, you might want to disable this feature. Leaving the feature enabled can cause OST files to become large, which can adversely affect Outlook performance when Cached Exchange Mode is used.

Cached Exchange Mode can be configured to download and synchronize the public folders included in users' Favorites folders for Outlook Public Folders. By default, Public Folder Favorites are not synchronized. However, you might want to enable this option if your organization uses public folders extensively. You can configure an option to download Public Folder Favorites in the OCT when you customize your Cached Exchange Mode deployment.

If users' Public Folders Favorites folders include large public folders, their OST files can also become large. This can adversely affect Outlook performance in Cached Exchange Mode. Before you configure Cached Exchange Mode to enable this option, ensure that users are selective about the public folders that are included in their Public Folder Favorites. Also ensure that users' OST files are large enough, and are in folders with enough disk space, to accommodate the additional storage requirements for the public folder downloads.

Outlook is configured to determine a user's connection speed by checking the network adapter speed on the user's computer, as supplied by the operating system. If the reported network adapter speed is 128 KB or lower, the connection is defined as a slow connection.

When a slow connection to a user's Exchange server is detected, Outlook helps users have a better experience by reducing the amount of less-critical information that is synchronized with the Exchange server. Outlook makes the following changes to synchronization behavior for slow connections:

  • Switches to downloading headers only

  • Does not download the Offline Address Book or OAB updates

  • Downloads the body of an item and associated attachments only when requested by the user

Outlook continues to synchronize with personal digital assistants (PDAs), and some client-side rules may run.

Synchronizing PDAs while using Cached Exchange Header Only Mode is not recommended. When you synchronize a hand-held computer—for example, by using ActiveSync—full items are downloaded in Outlook, and the synchronization process is less efficient than with regular Outlook synchronization to users' computers.

The Headers Only mode of synchronization is designed for Outlook users with dial-up connections or cellular wireless connections to minimize network traffic when there is a slow or expensive connection.

There might be circumstances when the network adapter speed does not accurately reflect data throughput for users. For example, if a user's computer is on a local area network for fast access to local file servers, the network adapter speed is reported as fast because the user is connected to a local area network (LAN). However, the user's access to other locations on an organization's network—including the Exchange server—might use a slow link, such as an ISDN connection. For a scenario like this, where users' actual data throughput is slow although their network adapters report a fast connection, you might want to configure an option to change or lock down the behavior of Outlook—for example, by disabling automatic switching to downloading only headers and configuring Outlook to download only headers. Similarly, there might be connections that Outlook has determined are slow in which users actually have high data throughput. In this scenario, you might also disable automatic switching to downloading only headers.

The setting you configure to change the behavior of Outlook for reported connection speed is the On slow connections, download only headers check box. You can configure this option in the OCT, or lock down the option by using Group Policy. For more information about customizing this setting, see Configure Cached Exchange Mode in Outlook 2007.

Stage the rollout over time if you plan to upgrade a large group of users from a deployment of Outlook without Cached Exchange mode (Outlook XP or earlier, or Outlook 2003 without Cached Exchange Mode) to Office Outlook 2007 with Cached Exchange Mode enabled. This helps your organization's Exchange servers manage the requirements of creating or updating users' OST files.

If most users are updated to use Cached Exchange Mode at once and then start Outlook at the same time (for example, on a Monday morning after a weekend upgrade), the Exchange servers will have significant performance issues.These performance issues can sometimes be mitigated; for example, if most of the users in your organization have current OST files. But in general, staging deployment of Cached Exchange Mode over a period of time is recommended.

The following scenarios include examples of how you could deploy Cached Exchange Mode to avoid a large initial performance impact on the Exchange servers and—in some cases—minimize the time users spend waiting for the initial synchronization:

  • Retain Outlook OST files while deploying Cached Exchange Mode.   Since existing OST files are merely updated with the latest mailbox information when Outlook with Cached Exchange Mode starts for the first time, retaining these files when you deploy Cached Exchange Mode can help reduce the load on your organization's Exchange servers. Users who already have OST files will have less Outlook information to synchronize with the server. This scenario works best when most users already have OST files that have recently been synchronized with Exchange Server. To retain OST files while you deploy Outlook with Cached Exchange Mode, do not specify a new Exchange server when you customize Outlook profile information in the OCT. Alternatively, when you customize Outlook profiles, clear the Overwrite existing Exchange settings if an Exchange connection exists (only applies when modifying the profile) check box.(If you specify an Exchange server when you configure and deploy Outlook with this option enabled, Outlook replaces the Exchange service provider in the MAPI profile, which removes the profile's entry for existing OST files.)

  • Provide seed OST files to remote users, and then deploy Cached Exchange Mode after users have installed the OST files you provide.   If most users in your organization do not currently have OST files or are not using Cached Exchange Mode, you can deploy Office Outlook 2007 with Cached Exchange Mode disabled. Then, before the date on which you plan to deploy Cached Exchange Mode, you provide initial or "seed" OST files to each user with a snapshot of the user's mailbox; for example, by providing or mailing to the user a CD that contains the file with installation instructions. You might also want to provide a recent version of your organization's Office Address Book (OAB) with Full Details. You configure and deploy Cached Exchange Mode when users confirm that they have installed the files.

For more information about creating initial OST files, see Providing an initial OST file for an Outlook Cached Exchange Mode deployment. The blog post describes creating initial OST files for Outlook 2013, but the process works similarly for Outlook 2007.

When you update your Outlook deployment to use Cached Exchange Mode later, the Exchange server updates users' existing OST files and there is much less data to synchronize than there would be if a new OST and OAB were created for each user. Creating individual CDs for each user's OST file can be time consuming, so this procedure might be most useful for select groups of remote users who would otherwise spend a lot of time waiting for the initial mailbox and OAB synchronization, perhaps at a high cost, depending on their remote connection scenario.

  • Deploy Outlook with Cached Exchange Mode to groups of users at a time.   You can balance the workload on your Exchange servers and the local area network by upgrading groups of users to Cached Exchange Mode over a period of time. The network traffic and server-intensive work of populating OST files with users' mailbox items and downloading the OAB are mitigated by rolling out the new feature in stages. The way that you create and deploy to groups of users depends on your organization's usual deployment methods. For example, you might create groups of users in Microsoft Systems Management Server (SMS), to which you would deploy an SMS package that updates Outlook to use Cached Exchange Mode. You would deploy SMS to each group over a period of time. To balance the load as much as possible, choose groups of users whose accounts are spread across groups of Exchange servers.

Upgrading users to Office Outlook 2007 with Cached Exchange Mode already enabled in Outlook 2003 is straightforward. If you do not change Cached Exchange Mode settings, the same settings are kept for Office Outlook 2007. There is no change to the OST or OAB file format, and you do not need to re-create these files during an upgrade.

The ability to share non-mail folders is a new feature that is enabled by default for Cached Exchange Mode in Office Outlook 2007. Existing profiles with Cached Exchange Mode have this setting enabled when users are upgraded. This could be problematic if:

  • Users in your organization use ANSI OST files.

  • Users' OST files are close to the size limit.

  • Your organization shares a large amount of data.

When these factors are all present, downloading shared non-mail folders can create performance issues and other problems.

You can disable this option when you deploy Outlook to help prevent problems with downloading non-mail folders.

In addition, be aware that caching for shared non-mail folders works differently from other caching for Cached Exchange Mode. With shared non-mail folders, replication to the local OST file starts only when the user clicks the shared folder. Once a user has activated caching for the folder by clicking it, Outlook updates the folder just like other Outlook folders are synchronized in Cached Exchange Mode. However, if the user does not navigate to the folder at least once every 45 days (the default value), the local data will be not be updated further until the user clicks the folder again.

You can configure the Synchronizing data in shared folders option in Group Policy or use the OCT to change the number of days before Outlook stops caching inactive non-mail folders. For more information about this setting, see Configure Cached Exchange Mode in Outlook 2007.

Some Microsoft Outlook users who connect to Microsoft Exchange in online mode might have OST files. There are several issues to consider when you configure Cached Exchange Mode for these users:

  • Users with large Exchange mailboxes   If users with existing OST files have large Exchange mailboxes, they might experience errors when Outlook attempts to synchronize their mailboxes to their OST files. To help prevent this, you can first configure a Group Policy setting that requires new Outlook files to be Unicode-formatted, since Outlook Unicode files do not have the 2-GB size limit that Outlook ANSI files do.

    Then, when Outlook is deployed with Cached Exchange Mode, Outlook creates a new Unicode OST file for users that currently have ANSI OST files. Users' existing OST and OAB files are not removed.

  • Users without a Full Details Offline Address Book (OAB)   For users who have not downloaded a Full Details Offline Address Book (OAB), a Full Details OAB is downloaded when Cached Exchange Mode synchronizes for the first time. Existing OAB files, including files for a No Details OAB, are not removed. Depending on several factors—including the version of Exchange Server you are using, your Exchange server Unicode settings, and the Outlook client Unicode settings—the new OAB files might be Unicode.

    If Unicode OAB files are created and users have ANSI OAB files (with Full Details or No Details), the ANSI OAB files are not removed.

If the Exchange Server version and settings support Unicode, you can require that new Outlook files are Unicode. For more information about configuring the default format for new Outlook files to be Unicode, see "To specify Unicode for new Outlook files" in Configure Cached Exchange Mode in Outlook 2007.

By using Group Policy, you can help prevent users from enabling Cached Exchange Mode in Outlook, enforce download options for Cached Exchange Mode, or configure other Cached Exchange Mode options.

For example, you can specify the default times between Exchange server synchronizations when data changes on an Exchange server. Those changes will be downloaded. You can also specify the default times when data changes on the client computer. Those changes will be uploaded. You can configure these options as defaults by using the Modify user settings page in the Office Customization Tool, or lock down the settings by using Group Policy.

Steps for locking down settings by using Group Policy are provided in Configure Cached Exchange Mode in Outlook 2007.

Refer to the resources listed below for additional information relevant to planning a Cached Exchange Mode deployment.

  • When you use Microsoft Office Outlook 2003 or 2nd_Outlook12 with Microsoft Exchange Server-based systems, you can use Cached Exchange Mode and other features to enhance the user experience regarding issues such as high latency, loss of network connectivity, and limited network bandwidth. Download the Client Network Traffic with Exchange 2003 white paper to learn about these new enhancements.

  • You can make changes to your configuration that improve the user experience in areas such as high latency, loss of connectivity, and limited bandwidth. For more information, download the Enabling a Superior Client Experience with Outlook 2003 whitepaper.

  • Office Outlook 2007 includes the ability to automatically configure user accounts. Download the Outlook Automatic Account Configuration whitepaper to learn how the discovery mechanisms work and how to modify an XML file to configure AutoDiscover for your organization.

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