Cached Exchange Mode in Outlook
Cached Exchange Mode gives classic Outlook a synchronized local Exchange or Microsoft 365 cache for offline access.
It is not the same as the separate OST storage used by IMAP accounts. New Outlook for Windows uses a different offline model, so the classic Outlook controls described on this page do not apply there.
Cached Exchange Mode in Outlook stores a local copy of your Exchange mailbox (OST file) on your computer. This allows you to access emails, calendar entries, contacts, and other mailbox items even when you’re offline. By reducing the need for constant server connections, it enhances performance and provides continued access to cached data during network disruptions.
This feature is particularly useful in areas with slow or unstable internet connections. If Outlook falls out of sync with the Exchange server, that's a synchronization issue rather than an OST recovery issue — see Outlook may fail to update for troubleshooting steps. OST recovery is relevant separately, when the OST file itself is damaged or the mailbox/profile relationship can no longer be restored.
Benefits of using the Cached Exchange Mode in Outlook
Cached Exchange Mode is available in supported classic Outlook for Windows configurations with Exchange or Microsoft 365 accounts. Its practical benefits depend on synchronization state, cache settings, network conditions, and mailbox size.
- Offline Access to your Mailboxes: One of the key advantages of Cached Exchange Mode is offline access to your mailbox. Since Outlook stores a local copy of your Exchange mailbox as an OST file, you can continue working with your emails, contacts, and calendar items even when there’s no internet connection.
- Improved Outlook Performance: Cached Exchange Mode enhances Outlook’s performance by minimizing server requests. When you access an item like an email, contact, or calendar entry, Outlook first checks the local cache for the latest version, reducing the need to repeatedly query the Exchange server for the same data.
- Reduced Dependence on Live Server Reads: Cached Exchange Mode serves many read operations from the local cache, which can reduce repeated live lookups; the overall server impact still depends on the Exchange environment and synchronization workload.
- Better Outlook Responsiveness: Cached Exchange mode can also result in faster item access and better responsiveness in Outlook, because the cached data is stored locally on your computer and is therefore quickly accessible.
- Improved Data Synchronization: Cached Exchange Mode also helps keep your Outlook emails in sync. Any changes you make while offline like sending messages or updating calendar events, are automatically synchronized with the Exchange server the next time Outlook connects.
Limitations of Outlook Cached Exchange Mode
While Cached Exchange mode provides several benefits to Outlook users, there are also some limitations to be aware of:
- Increased Disk Space on the Computer: Storing a copy of the Exchange mailbox on your computer requires a significant amount of disk space, which can be a concern if you have limited storage on your computer.
- Delayed Data Updates and Slower Performance: While Cached Exchange Mode offers offline access, it can sometimes delay updates to mailbox data. Changes made directly on the Exchange server might not appear in Outlook until synchronization occurs. Additionally, when setting up a large mailbox for the first time, Outlook may run slower during the initial sync process. If sync delays persist beyond normal catch-up, see Fix Outlook Not Syncing — converting the OST to PST does not resolve a live synchronization problem.
- Local cache size: Cached Exchange Mode creates an OST whose size depends on the selected cache range and locally synchronized content. Large caches can affect client performance or reach configured limits, but size alone does not prove corruption. A temporary server outage also does not orphan the OST; synchronization normally resumes when connectivity returns.
- Limited Functionality: Cached Exchange mode may also have limited functionality compared to working directly with the Exchange server. Public folders are not cached by default, though Outlook supports caching Public Folder Favorites when that option is configured — see Shared Folders and Public Folder Favorites below.
Differences Between Cached Mode and Online Mode
Cached Exchange Mode saves a local version of your Exchange mailbox on your system, making your mailbox data available directly from your computer. As a result, you can view and work with emails, contacts, calendar items, and other mailbox content even without an active internet connection. Any updates or changes made while offline are automatically synchronized with the Exchange server once connectivity is restored.
The Online mode, on the other hand, is a mode in which all data is stored on the Exchange server and accessed by connecting directly to the server in real-time. You do not have offline access to your mailbox in this mode and must have an active Internet connection to access the mailbox data. The following table illustrates the difference between the two modes.
| Feature | Cached Exchange Mode | Online Mode |
|---|---|---|
| Offline Access | Yes | No |
| Performance | Depends on sync state and local cache | Depends on network and server conditions |
| Server Load | Reduced | Increased |
| Responsiveness | Local cache accessed directly; freshness depends on last sync | Server view is immediate when connected |
| Data Synchronization | Background synchronization | Direct server access while connected |
| Disk Space | Increased | No Impact |
| Functionality | Full mailbox access; some settings (folder permissions, rules) are managed server-side | Full mailbox access; server view is immediate |
Cached Exchange Mode: What Is and Is Not Stored Locally
| Mailbox content | May be in the OST? | How to verify |
|---|---|---|
| Recent primary mailbox items | Usually, according to the configured cache range. | Compare an older item in Outlook and Outlook on the web. |
| Older server content | Not necessarily. | Check the cache range and server mailbox. |
| Online Archive | Not necessarily in the primary OST. | Check the archive separately in its configured source. |
| Shared mailboxes/folders | Depends on Outlook settings and synchronization. | Verify the relevant shared folders before profile changes. |
| Unsynchronized local changes | Potentially, but not guaranteed. | Preserve the old OST before recreating it. |
Before Reducing Cache or Rebuilding the OST
- Confirm the server mailbox contains the data you need.
- Record the current cache range and shared-folder settings.
- Preserve an old OST if there may be unsynchronized local information.
- Do not use the OST as a substitute for a formal backup.
How to know if Outlook is running in cached mode?
To determine whether Microsoft Outlook is running in Cached Exchange mode or Online mode, follow these steps:
- Step 1: Open Outlook and go to Account Settings
- Start Outlook, go to ‘File’ > ‘Account Settings’ > ‘Account Settings’
- In the Account Settings window, select the email account you want to check and click ‘Change’.
- Start Outlook, go to ‘File’ > ‘Account Settings’ > ‘Account Settings’
- Step 2: Access More Settings for your email account
- In the Change Account window, click ‘More Settings’.
- In the Microsoft Exchange dialog box, go to the ‘Advanced’ tab.
- Step 3: Check the Cached Exchange mode setting
- In the ‘Advanced’ tab, look for the option ‘Use Cached Exchange Mode’
- If the checkbox is selected, Outlook is running in Cached Exchange mode.
- If the checkbox is unselected, Outlook is running in Online mode.
What does disabling the cache mode in Outlook do?
Disabling the Cached Exchange mode in Microsoft Outlook means that the application no longer stores a copy of your Exchange mailbox on the local computer. Instead, you will have to access the mailbox directly on the Exchange server in real-time, without any local cache.
Disabling the cache mode stops Outlook from maintaining the local OST copy going forward; it does not by itself guarantee the existing OST file is immediately deleted or that its disk space is freed right away, and changes made to items in the mailbox will be reflected in real-time. However, disabling the cache mode may result in slower performance and responsiveness in Outlook, as the application will no longer have a local copy of the mailbox data to work with. Additionally, if you have poor Internet connectivity, you may experience issues accessing your mailbox data.
How to turn off Cached Exchange mode in Outlook 365?
The following steps apply to supported classic Outlook for Windows builds; labels can vary by build and organizational policy.
Note: These steps may vary slightly in different versions of Outlook, but the general process for turning off Cached Exchange mode should be similar in most cases.
- Step 1: Open Account Settings and select the email account
- Open Outlook and go to ‘File’ > ‘Account Settings’ > ‘Account Settings’.
- In the Account Settings window, select the email account for which you want to turn off Cached Exchange mode and click ‘Change’.
- Step 2: Access More Settings in Outlook and click Advanced
- In the Change Account window, click the ‘More Settings’ button.
- Open Microsoft Exchange Dialog Box: In the Microsoft Exchange dialog box, go to the ‘Advanced’ tab.
- Step 3: Disable the Cached Exchange Mode Setting
- In the ‘Advanced’ tab, uncheck the ‘Use Cached Exchange Mode’ checkbox.
- Click ‘OK’ to save the changes and close the dialog boxes.
- Close Microsoft Outlook and then restart the application for the changes to take effect.
How to turn on Cached Exchange mode in Outlook?
To enable Cached Exchange Mode in supported classic Outlook for Windows, follow these steps:
- Step 1: Open Account Settings in Outlook
- Go to ‘File’ > ‘Account Settings’ > ‘Account Settings’.
- Step 2: Select the email account and access more settings
- In the Account Settings window, select the email account for which you want to turn on Cached Exchange mode and click ‘Change’
- In the Change Account window, click the ‘More Settings’ button.
- In the Microsoft Exchange dialog box, go to the ‘Advanced’ tab.
- Step 3: Enable the Cached Exchange Mode Setting
- In the ‘Advanced’ tab, check the ‘Use Cached Exchange Mode’ checkbox.
- Click ‘OK’ to save the changes and close the dialog boxes.
- Close Microsoft Outlook and then restart the application for the changes to take effect.
Shared folders cached mode
Classic Outlook can cache shared mail folders when Download Shared Folders is enabled, and can separately cache Public Folder Favorites. Caching a large shared mailbox increases the local OST size and can affect synchronization or client performance, so enable only what the user needs.
- Step 1: Open Outlook and go to Account Settings:
- Start Outlook on your computer
- Go to ‘File’ > ‘Account Settings’> ‘Account Settings’
- Step 2: Select the Email Account and go to more settings
- In the Account Settings window, select the email account for which you want to turn on Cached Exchange mode for shared folders and click ‘Change’.
- In the Change Account window, click the ‘More Settings’ button.
- In the Microsoft Exchange dialog box, go to the ‘Advanced’ tab.
- Step 3: Enable the Cached Exchange mode for Shared folders
- In the ‘Advanced’ tab, select the ‘Use Cached Exchange Mode’ option.
- Select the option ‘Download Shared Folders’, which enables Cached Exchange mode for the shared folders that are associated with the email account.
- You can also enable ‘Download Public folder Favorites’, if you want to keep a copy of them.
- Click ‘OK’ to save the changes and close the dialog boxes.
- Close Microsoft Outlook and then restart it for the changes to take effect.
When EdbMails OST to PST Converter Is Relevant
An OST (Offline Storage Table) file is a local copy of your Exchange mailbox, created when Cached Exchange Mode is turned on in Outlook. A temporary Exchange server outage doesn't normally cause this — Cached Mode continues working from local data and re-syncs once the server is reachable. The OST becomes a genuine recovery problem when the file itself is damaged (for example after sudden system shutdowns or storage errors) or when the mailbox/profile relationship can no longer be restored, such as after a deleted profile or mailbox. In those cases, Outlook cannot reconnect the OST to a live mailbox.
The EdbMails OST to PST Converter can scan the old OST and export readable data to PST when the live mailbox or profile can no longer provide it. Recoverability depends on what is present and readable in the source file.
EdbMails can scan and preview the old OST without the original Outlook profile. Outlook with a configured profile is required for PST export; direct migration to Exchange or Microsoft 365 does not require Outlook.
Frequently Asked Questions
Is Cached Exchange Mode available in new Outlook?
The Cached Exchange Mode controls described on this page apply to classic Outlook. New Outlook uses a different synchronization and offline-access model.
Can IMAP accounts use Cached Exchange Mode?
No. Although classic Outlook may store synchronized IMAP data in an OST file, Cached Exchange Mode is designed for Exchange Server and Microsoft 365 accounts.
How much email is stored in the OST file?
The amount depends on the Mail to keep offline setting, the synchronization status and the folders cached locally. A separate Online Archive is not stored in the primary OST file.
Are shared folders and Public Folder Favorites cached?
Shared mail folders can be cached when Download Shared Folders is enabled. Public Folder Favorites are cached separately when their corresponding download option is enabled.
Does turning off Cached Exchange Mode delete the OST file?
Not necessarily. Turning off Cached Exchange Mode stops Outlook from maintaining the offline cache, but it does not guarantee that the existing OST file will be deleted immediately or that disk space will be recovered automatically.
Does temporary Exchange Server downtime make an OST file orphaned?
No. A temporary server outage normally pauses synchronization. Outlook should resume synchronization and update the OST file after the Exchange Server becomes available again.
