Choose between Cached Exchange Mode and Online Mode for Outlook 2013
Published: October 16, 2012
Summary: Learn about the two Exchange connectivity modes that you can choose to use for Outlook 2013 deployments.
Applies to: Office 365 ProPlus | Outlook 2013
Audience: IT Professionals
Outlook 2013 offers two basic connectivity modes when you are connected to an Exchange Server computer: Cached Exchange Mode or Online Mode. This article discusses which connectivity mode might be appropriate for your environment.
In this article:
Overview of Cached Exchange Mode and Online Mode
When you configure an Outlook 2013 account to use Cached Exchange Mode, Outlook 2013 works from a local copy of a user's Microsoft Exchange mailbox that is stored in an offline data file (.ost file) on the user's computer, together with the Offline Address Book (OAB). The cached mailbox and OAB are updated periodically from the Exchange Server computer.
Cached Exchange Mode was introduced in Outlook 2003 to provide users a better online and offline experience. Cached Exchange Mode lets users move between connected and disconnected environments without interrupting their experience in Outlook. Also, it insulates users from network latency and connectivity issues while they are using Outlook.
In contrast, Online Mode works by using information directly from the server. When new information is required in Outlook, a request is made to the server and the information is displayed. Mailbox data is only cached in memory and never written to disk.
The user can select Cached Exchange Mode or Online Mode during account setup or by changing the account settings. You can also deploy the mode by using the Office Customization Tool (OCT) or Group Policy.
Choosing between Cached Exchange Mode and Online Mode
When to use Cached Exchange Mode
Cached Exchange Mode is the premier configuration in Outlook 2013. We recommend it in all circumstances, except those specifically indicated in When to use Online Mode later in this article.
Although we recommend Cached Exchange Mode in most user configurations, it is especially valuable for the following types of users:
Portable computer users who frequently move in and out of connectivity.
Users who frequently work offline or without connectivity.
Users who have high-latency connections (greater than 500ms) to the Exchange Server computer.
When to use Online Mode
Online Mode is the legacy method of connecting to Microsoft Exchange. It is a fully supported configuration in Outlook 2013. Online Mode has value in certain scenarios in which the behavior of Cached Exchange Mode is unwanted. The following scenarios are examples:
“Kiosk” scenarios in which a particular computer has many users who access different Outlook accounts and the delay to download e-mail messages to a local cache is unacceptable.
Heavily regulated compliance or secure environments in which data must not ever be stored locally. In these environments, we recommend that you evaluate Encrypting File System (EFS) or BitLocker in addition to Cached Exchange Mode as a potential solution.
Very large mailboxes on computers that have insufficient hard disk space for a local copy of the mailbox.
Very large mailboxes (greater than 25 GB) on which performance considerations become an issue in Cached Exchange Mode.
Virtualized or Remote Desktop Services (Terminal Services) environments that run Outlook 2013 and on which disk size or disk input/output (I/O) limitations prevent running Cached Exchange Mode at the desired scale.
If you work with a very large mailbox, you can reduce the size of the local data file by using Cached Exchange Mode with the Exchange Fast Access Sync Slider enabled. Sync Slider allows an Outlook 2013 user to limit the email messages that are synchronized locally in an Outlook data file (.ost). By default, if Cached Exchange Mode is enabled, Outlook 2013 will only cache email messages from the last 12 months and it will remove anything older than 12 months from the local cache. The email messages that are removed from the local cache will still be available for users to view. Users can scroll to the end of an email list in a folder and then choose the message Click here to view more on Microsoft Exchange to view the email messages that were removed. For more information, see Synchronization, disk space, and performance considerations for Cached Exchange Mode.
If you work with a very large mailbox on which performance considerations become an issue in Cached Exchange Mode, see How to troubleshoot performance issues in Outlook.
Special considerations for Cached Exchange Mode
Outlook 2013 supports running in Cached Exchange Mode in a Remote Desktop Services (Terminal Services) environment that has multiple users. When you configure a computer that is running Remote Desktop Services (Terminal Services) to use Cached Exchange Mode, you must consider the additional storage space and disk I/O that are required for multiple client access.
By default, new Exchange accounts that are set up on a computer that is running Remote Desktop Services (Terminal Services) use Online Mode. Upon setup, the user can decide to enable Cached Exchange Mode or this setting can be controlled by using the Use Cached Exchange Mode for new and existing Outlook profiles option in the Office Customization Tool or Group Policy.
In very limited bandwidth environments, Cached Exchange Mode can be configured to download only e-mail headers and a 256-character preview of the message body. For more information, see Configure Cached Exchange Mode in Outlook 2013.
Even when it is configured in Cached Exchange Mode, Outlook 2013 must contact the server directly to do certain operations. These operations do not function when Outlook is not connected and can take longer to complete on high-latency connections. These operations include the following:
Working with Shared Folders that were not made available offline. For more information, see Configure Offline Availability for a Shared Folder.
Retrieving Free/Busy information
Setting, changing, or canceling an Out of Office message
Accessing Public Folders
Retrieving rights to a rights-protected message
Retrieving Policy Tips
How Cached Exchange Mode can help improve the Outlook user experience
Use of Cached Exchange Mode provides the following key benefits:
Shields the user from network and server connection issues.
Facilitates switching from online to offline for mobile users.
By caching the user's mailbox and the OAB locally, Outlook no longer depends on continuous network connectivity for access to user information. While connected, Outlook continuously updates users’ mailboxes so that the mailboxes are kept up-to-date. If a user disconnects from the network, for example, by removing a portable computer, such as a laptop, from a docking station, the latest information is automatically available offline.
In Outlook 2013, Exchange Fast Access is a new feature that is available with Cached Exchange Mode. It combines the instant access of Online Mode with the offline capabilities and syncing robustness of Cached Exchange Mode. When Cached Exchange Mode is enabled, and users first start Outlook 2013, they immediately see their most recent email messages and up-to-date calendar information as if they are in Online Mode. Outlook 2013 caches a local copy the user’s mailbox in the background to prepare the user for offline use without affecting the user's experience. This is especially helpful in scenarios when syncing data locally would take enough time to be noticed by the user (for example, initial sync, resume, returning from vacation).
Besides using local copies of mailboxes to improve the user experience, Cached Exchange Mode optimizes the type and amount of data that is sent over a connection with the server. For example, if the On slow connections, download only headers setting is configured in the Office Customization Tool, 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 kilobytes (KB) or lower are defined as slow connections. Under some circumstances, the network adapter speed might not accurately reflect data throughput for users. For more information about adjusting the behavior of Outlook in these scenarios, see Managing Outlook behavior for perceived slow connections in Plan a Cached Exchange Mode deployment in Outlook 2013.
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 re-establishing a connection to the server over a slower, dial-up connection. When the Exchange Server connection type changes, such as LAN, wireless, cellular, or offline connections, transitions are seamless and do not require changing settings or restarting Outlook.
For example, a user might have a portable computer at work that has a network cable connection to a corporate LAN. In this scenario, the user has access to headers and full items, including attachments. The user also has quick access and updates to the computer that runs Exchange Server. If a user disconnects the portable computers from the LAN, Outlook switches to Trying to connect mode. The user can continue to work uninterruptedly by using the data in Outlook. If a user has wireless access, Outlook can re-establish a connection to the server and then switch back to Connected mode.
If the user later connects to the Exchange Server computer over a dial-up connection, Outlook recognizes that the connection is slow and automatically optimizes for that connection by downloading only headers and by not updating the OAB. In addition, Outlook 2013, Outlook 2010 and Office Outlook 2007 include optimizations to reduce how much data that is sent over the connection. The user does not have to change settings or restart Outlook in this scenario.
Outlook 2013 also includes the Need Password mode. A Need Password message is displayed when Outlook is in a disconnected status and requires user credentials to connect. For example, when a user chooses Cancel in a credentials authentication dialog box. When Outlook is disconnected but is not offline, a user-initiated action (such as choosing Send/Receive or the Type Password button on the ribbon) causes Outlook to prompt again for the password and to display a Trying to connect message until the user can successfully authenticate and connect.
Outlook features that can reduce the effectiveness of Cached Exchange Mode
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.
The following features might rely on network access and can cause delays in Outlook unless users have fast connections to Exchange Server data:
Delegate access, when folders are not cached locally (local cache is the default).
Opening another user's calendar or folder that is not cached locally (local cache is the default).
Using a public folder that is not cached.
For more information, see Managing Outlook folder sharing in Plan a Cached Exchange Mode deployment in Outlook 2013.
We recommend that you disable or do not implement the following features, or combination of features, if you deploy Cached Exchange Mode:
The toast notification that has digital signatures on email messages Outlook must check a server to verify a digital signature. By default, when new messages arrive in a user's Inbox, Outlook displays a toast notification that contains a part of an email message. If the user chooses the toast notification to open a signed email 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 have to access the network to check these address books every time that a name is resolved in an email message that a user is composing.
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 the Download only headers and On slow connections, download only headers settings in Cached Exchange Mode. For example, full Outlook items, not only 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-time-only applications use a less-efficient kind of synchronization.