What is Cached Exchange Mode in Outlook? (Enable, Configure & Troubleshoot)

Cached Exchange Mode is a feature within Microsoft Outlook designed to enhance the user experience when working with an Exchange Server account. It creates a local copy of your entire mailbox as an OST - Offline Storage Table - file on your computer’s hard drive — typically at C:\Users\[username]\AppData\Local\Microsoft\Outlook\.

Outlook introduced Cached Exchange Mode in 2003. Since then, it has been the default for Exchange and Microsoft 365 accounts. Even with slow or unstable internet connections, it allows you to work smoothly. Any changes you make, such as reading mail, sending messages, or updating contacts, Outlook saves those changes locally and syncs them with the server once you are connected to the internet.

The size of your OST file is usually 50–80% larger than the actual server mailbox because it stores additional data such as attachments, metadata, search indexes, and GAL – Global Address List.    

Read this Microsoft document for more details. 

Cached Exchange Mode vs Online Mode — Key Differences

Below, we have mentioned the key differences between Cached Exchange Mode and Online Mode in Outlook. It helps you to choose the most suitable option as per your work environment and storage requirements.

Key Advantages of using Cached Exchange Mode

Cached Exchange Mode helps improve Outlook performance and ensures uninterrupted access to mailbox data. Some of the benefits of this mode are:

Offline Access to your Full Mailbox

It allows you to access your entire mailbox data, such as emails, calendar etc., without an internet connection. 

Improved Outlook Performance

In this mode, Outlook stores all your mailbox data locally in the OST file, resulting in faster email access and quicker folder loading.

Faster Email Search

It enhances search speed because Outlook searches the local cache instead of querying the server repeatedly.

GAL Cached Locally

The Global Address List is stored in the OST file. This lets you find email addresses and contacts across your organization without needing a server connection.

Shared Folders and Public Folders are Downloaded

Outlook downloads shared or public folders to the OST file. You can disable this by going to Account Settings > More Settings > Advanced and then unchecking the Download Shared Folders option.

Minimizes Load on the Exchange Server

It minimizes load on the Exchange Server by handling routine mailbox operations locally.

Built-in Content Index

Outlook builds a local search index from the OST file, making full-text search fast and reliable without server interaction.

Limitations and When not to Use Cached Exchange Mode

Although Cached Exchange Mode helps improve Outlook performance, it also has some limitations, such as:

1. Local Disk Space Required

In this mode, Outlook stores your mailbox data locally on your device. At times, this file grows 50–80% larger than your server mailbox because of locally stored metadata, attachments, and search index. If your device has limited storage capacity — such as Ultrabook or tablet — it may struggle to handle large OST file. In situations like this, you should use Online Mode.

2. Risk of OST File Corruption

If your OST has reached the stipulated size limit, it may get corrupt or become inaccessible. In such a case, you can use a Stellar Converter for OST to recover the mailbox items.

3. Downloading Shared Mailbox Increases OST File Size

If you have delegate access permissions or access to shared mailboxes and Public Folders, Outlook downloads that data into the OST file by default. It significantly increases its size and consumes additional disk space.

4. Search Indexing Issues

Large OST files can cause Outlook indexing problems. Users may experience:

• Missing search results
• Endless “Indexing in progress” messages
• Slow searches
• Outlook freezing issue

These issues can also lead to the Outlook out of memory error in some systems.

5. New Outlook (2024) handles CEM Differently

If you are using the New Outlook for Microsoft 365, the settings path is different. See the New Outlook section below. 

Cached Exchange Mode Sync Slider (Mail to Keep Offline)

Sync Slider — also called Mail to Keep Offline – is a useful feature that controls how much mailbox data Outlook downloads into the OST file. By default, Outlook is set to download all mail items. If you are managing a large mailbox, it can cause your Offline Storage Table – OST – file to grow large.  Reducing the sync duration is one of the most effective ways to decrease its size without disabling Cached Exchange Mode.

How to Adjust the Sync Slider in Outlook 2016 / 2019 / 2021 / 365 (Classic Outlook)?

Follow the steps below:

• Open Outlook.
• Go to File > Account Settings > Account Settings.
• Select your Exchange or Microsoft 365 account from the list.
• Then, click Change.

• Locate the 'Mail to keep offline' slider.
• Drag the slider to select a time period: 1 month to 24 months, or All.

• Click Next > Finish. Then, restart Outlook.

You can refer the table below for recommended settings based on your situation.

For more details, read What is Outlook Cached Mode Sync Slider.

How to Enable or Disable Cached Exchange Mode?

When you configure your email account with an Exchange or Microsoft 365 account in Outlook 2013 and later versions, Cached Exchange Mode is enabled by default. However, you can enable or disable it any time by following the steps below:

1. Open Outlook.
2. Go to File > Account Settings > Account Settings.

1. Select your Exchange or Microsoft 365 account.
2. Click Change.

1. Check Use Cached Exchange Mode to enable it.
2. Uncheck the option to disable it.

1. Click Next and then Finish.
2. Restart Outlook.

New Outlook (2024 Redesign)

New Outlook does not provide the traditional Cached Exchange Mode settings for windows. The "Use Cached Exchange Mode" checkbox and the traditional "Mail to keep offline" sync slider are features of Classic Outlook.

Instead New Outlook automatically manages local data synchronization and offline access in the background.

Note: New Outlook uses a different local caching mechanism than Classic Outlook. It does not create a traditional Outlook Offline Storage Table (.ost) file that users can manage directly. Instead, it stores synchronized mailbox data in a proprietary local data store managed by the application.

Outlook 2010

If you are using Outlook 2010, then follow the steps below: 

• Open Outlook 2010.
• Go to File.
• Click Account Settings > Account Settings.
• Select your Exchange account.
• Click Change.
• Check or uncheck Use Cached Exchange Mode.
• Click Next.
• Restart Outlook.

Enable Cached Exchange Mode via Group Policy (For IT Admins)

IT administrators can configure this organization-wide using Group Policy and Outlook ADMX templates. Below are the steps to configure Cached Exchange Mode via GPO:

• Download the Office ADMX/ADML templates from Microsoft.
• Open Group Policy Management Console (GPMC).
• Navigate to User Configuration.
• Click on Administrative Templates > Microsoft Outlook .
• Select Account Settings.
• Click Exchange > Cached Exchange Mode.

• In the Exchange reading pane, open Use Cached Exchange Mode for new and existing Outlook profiles.
• Select Enabled and click OK.

Troubleshooting Common Cached Exchange Mode Issues

Below, we have mentioned some common Cached Exchange Mode issues and the steps to solve it. 

1) Outlook not Syncing in Cached Exchange Mode

If your emails or mail folders are not updating in cache mode, calendar events are missing, or sent items are not appearing, then try the troubleshooting steps given below:

• Check the Outlook status bar at the bottom of the window. If it displays “Disconnected” or “Work Offline” then:
  • Go to File > Info.
  • Check whether Outlook is connected to the mail server.
• Press F9 to try syncing Outlook manually or click Send/Receive > Send/Receive All Folders.

• You can check the size of your OST file. If it is larger than 20–25 GB, then reduce the sync slider to 3–6 months. 
• Close and reopen Outlook.
• Also, check for Office updates by:

• Going to File > Office Account.
• Click on Update Options > Update Now.

2) OST File Grows too Large

If your OST file size has increased, it can cause Outlook performance, sync, or indexing issues. You can resolve this issue by following the methods below:

• Reduce the Sync Slider to 3–6 months to cache fewer emails locally.
• Disable shared folder downloads by:

• Navigating to Account Settings.
• Clicking on More Settings > Advanced.

• Archive older mailbox items to a PST file by:

• Going to File > Cleanup Tools > Archive.

• Compact the OST file by:

• Going to Account Settings > Data Files
• Selecting the OST file.
• Clicking Compact Now.

3) Cached Exchange Mode Content Index Stopped Working

If Outlook search keeps showing 'Outlook is currently indexing your items' message for a long time or returns incomplete results, then:

• Go to File > Options > Search > Indexing Options. Click Modify and confirm that Microsoft Outlook is listed as an indexed location.
• Reduce the OST file size if it is over 20 GB and then rebuild the index. For this, go to Indexing Options > Advanced > Rebuild.

4) Cached Exchange Mode Option is Greyed Out

If the “Use Cached Exchange Mode” option is not available, or is greyed out, in Outlook, it means that the “Use Cached Exchange Mode” option was disabled by your organization's IT administrator via Group Policy. You can contact your IT admin to verify and/or modify the GPO settings. If you're running your own Outlook profile, however, then follow the procedure to resolve the Cached Exchange Mode greyed out issue.

Also, if CEM is enabled, but your Outlook is not synching properly, you may try the local cache option. It will make Outlook generate a new OST file and obtain a new copy of the mailbox file. See: how to reset the local cache in Outlook.

When to Use Cached Exchange Mode or Online Mode?

Use the table below to decide which mode is right for your situation:

Conclusion

Cached Exchange Mode keeps a local copy of your mailbox on your device. This lets you read or write emails even without internet. You can turn this mode on or off anytime after you have set up your Exchange account in Outlook.

If your OST - Offline Storage Table - file becomes inaccessible due to large size or any other reason, you can use a reliable OST to PST converter tool, such as Stellar Converter for OST. This utility allows you to convert the inaccessible, damaged, or orphaned Outlook data file to PST format. This software also exports OST data directly to Microsoft 365 (Office 365) or Live Exchange.