Choose between Cached Exchange Mode and Online Mode for Outlook 2013


Applies to: Office 365 ProPlus, Outlook 2013

Summary: Learn about the two Exchange connectivity modes that you can use for Outlook 2013 deployments—Cached Exchange Mode and Online Mode.

Audience: IT Professionals

Decide which connectivity mode, Cached Exchange Mode or Online Mode, is appropriate for your environment. 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 gives users offline access.

The user can select Cached Exchange Mode or Online Mode during account setup or by changing the account settings. You can also deploy one mode or the other by using the Office Customization Tool (OCT) or Group Policy.

Are you a user?

If you’re not an administrator, this article is not for you. But we can point you in the right direction if you want to learn how to turn on Cached Exchange Mode in Outlook 2013 or change how much email to keep offline.

Are you an admin?

If you’re an administrator, this article tells you about Cached Exchange Mode and Online Mode and helps you decide when to deploy each. If you decide to deploy Cached Exchange Mode, learn how to Configure Cached Exchange Mode.

In this article:

  • Overview of Cached Exchange Mode

  • Choosing between Cached Exchange Mode and Online Mode

  • How Cached Exchange Mode can help improve the Outlook user experience

  • Outlook features that can reduce the effectiveness of Cached Exchange Mode

Overview of Cached Exchange Mode and Online Mode

Cached Exchange Mode was introduced in Outlook 2003 to give users a seamless online and offline Outlook experience. It also insulates users from network latency and connectivity issues while they are using Outlook. By caching the user's mailbox and the OAB locally with Cached Exchange Mode, 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 Outlook information is automatically available offline.

Online Mode works by using information directly from the server, and, as the name implies, it requires a connection. Mailbox data is only cached in memory and never written to disk.

Choosing between Cached Exchange Mode and Online Mode

When to use Cached Exchange Mode

Cached Exchange Mode is the preferred configuration in Outlook 2013.

Cached Exchange Mode is especially valuable for:

Users who move in and out of connectivity

Portable computer users who frequently move in and out of connectivity.


Users who frequently work offline or without connectivity.

Slow connection

Users who have high-latency connections (greater than 500 ms) to the Exchange Server computer.

When to use Online Mode

Online Mode is a fully supported configuration in Outlook 2013 and is the recommended configuration in the following situations:


Kiosk scenarios, where a particular computer has many users who access different Outlook accounts—and the delay to download email messages to a local cache is unacceptable.

Secure environments

Heavily regulated compliance or secure environments where data must not ever be stored locally. In these environments, we also recommend that you consider using Encrypting File System (EFS) or BitLocker with Cached Exchange Mode as a robust solution.

Very large mailboxes

Large mailboxes on computers that don’t have sufficient hard disk space for a local copy of the mailbox.

Large mailboxes (greater than 25 GB). For large mailboxes, performance can become an issue in Cached Exchange Mode.

RDS environments with disk limitations

Virtualized or Remote Desktop Services (RDS) (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 scale you want.

If users work with a large mailbox, you can reduce the size of the local data file by using Cached Exchange Mode with the sync slider enabled. The sync slider lets Outlook 2013 users limit the email messages that are synchronized locally in their Outlook data file (.ost). By default, if Cached Exchange Mode is enabled, Outlook 2013 will cache email messages only from the last 12 months and remove anything older than 12 months from the local cache. The email messages that are removed from the local cache are still available for users to view, but they’ll need to be connected to the Exchange Server computer to view them. Users can scroll to the end of the 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 from the local cache. This is just one way to improve the performance of Cached Exchange Mode for users.

What about Cached Exchange Mode in a Remote Desktop Services environment?

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 running RDS to use Cached Exchange Mode, be sure to consider the additional storage space and disk I/O that are required for multiple client access.

New Exchange accounts set up on computers running RDS use Online Mode by default. At setup, the user can decide to enable Cached Exchange Mode, or the user can control this setting by using the Use Cached Exchange Mode for new and existing Outlook profiles option in the Office Customization Tool or in Group Policy.

What Outlook operations always need to contact the Exchange server?

Even when it is configured in Cached Exchange Mode, Outlook 2013 must contact the server directly to do certain operations. These operations won’t function when Outlook is not connected and can take longer to complete on high-latency connections. These operations include:

  • Working with Shared Folders that were not made available offline. If your users take advantage of the Outlook 2013 Shared Folder features, you might want to configure shared folders for offline availability.

  • Retrieving Free/Busy information.

  • Setting, changing, or canceling an Out of Office message.

  • Accessing public folders that were not made available offline.

  • Retrieving rights to a rights-protected message.

  • Editing rules.

  • Retrieving MailTips.

  • Retrieving Policy Tips.


Delayed delivery options are client side in cached mode and server side in online mode. So, when using Cached Exchange Mode, Outlook must be connected and open at the assigned delivery time for the delayed delivery message to be sent.

How Cached Exchange Mode can help improve the Outlook user experience

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).

Connection optimizations happen automatically

Cached Exchange Mode also 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 KB or lower are defined as slow connections. Under some circumstances, the network adapter speed might not accurately reflect data throughput for users. You might want to see what you can do to manage Outlook behavior for perceived slow connections while planning your Exchange 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 to 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, uninterrupted, 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 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 state 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 (like choosing Send/Receive or the Type Password button on the ribbon) causes Outlook to prompt again for the password and 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 they bypass Cached Exchange Mode functionality.

The following features might rely on network access and can cause delays in Outlook unless users have fast connections to Exchange Server data:

  • Delegating 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.

You’ll probably want to look into Outlook folder sharing considerations when you plan your Exchange 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 also 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.

See also

Plan a Cached Exchange Mode deployment in Outlook 2013
Configure Cached Exchange Mode in Outlook 2013