How to Clear Cache in Outlook 365 on Windows, Mac, and Web

To clear the Outlook 365 cache, first choose the version you use: classic Outlook for Windows can rebuild its offline mailbox file, new Outlook for Windows can be repaired or reset, and Outlook on the web uses your browser’s site data. On a Mac, you can empty a folder’s local cache in supported versions. These actions refresh locally stored information; they do not normally delete mail kept on your Microsoft 365 server. Before making changes, let any unsent messages finish sending and check that important mail is visible online.

What You’ll Need

Access to the Outlook account and, if prompted, its sign-in or multi-factor authentication details.
A reliable internet connection so Outlook can download mailbox data again.
For a Windows desktop repair, permission to change app settings or locate Outlook data files. Close Outlook before renaming any data file.
For a browser-based fix, the browser where Outlook on the web is having trouble. Clearing Outlook site data may sign you out.

Step-by-Step

Identify which Outlook you are using. Classic Outlook for Windows has a File tab in its ribbon. New Outlook for Windows looks more like Outlook on the web and generally does not have the same File > Account Settings workflow. Outlook on the web runs in a browser, while Outlook for Mac is a separate desktop app. Choose the matching instructions below; a method for one version may not exist in another.
Start with a low-risk refresh in classic Outlook for Windows. Open Outlook and select the folder that is not updating. Right-click the folder, choose Properties, and, if the option is available, open the General tab and select Clear Offline Items. This removes the folder’s locally stored copy, not the server copy. Then select the Send/Receive tab and choose Update Folder. Outlook downloads the folder again, which can take time for a large mailbox. If you do not see Clear Offline Items, move to the next step rather than searching for a similarly named command in another version.
Rebuild the offline mailbox file in classic Outlook, if needed. Use this for a Microsoft 365 or Exchange account when Outlook’s local mailbox copy appears damaged or stuck. In Outlook, go to File > Account Settings > Account Settings, then select the Data Files tab. Select the data file associated with the Microsoft 365 mailbox and choose Open File Location. Confirm that the selected file has an .ost extension. Do not rename or delete a .pst file: PST files can contain mail or other items stored only on your computer.
Close Outlook and rename the OST file. Exit Outlook completely. If it remains open, use Task Manager to check that Outlook is no longer running. In the File Explorer window opened in the previous step, right-click the correct OST file, choose Rename, and add a suffix such as .old to its name. For example, change mailbox.ost to mailbox.old.ost. Renaming is easier to reverse than deleting. Do not do this to an OST for an account that may contain unsynced or locally stored information; verify that needed items are available on the server first.
Open Outlook and let it synchronize. Outlook should create a new OST and download the mailbox from Microsoft 365. Keep the computer online and leave Outlook open. A large mailbox, attachments, shared folders, or a slow connection can make the first synchronization lengthy. Check that recent messages and folders appear before removing the renamed file. If Outlook does not recreate the file, see Troubleshooting below.
Repair or reset new Outlook for Windows. Close Outlook. In Windows, open Settings > Apps > Installed apps, find Outlook (new) (the label may vary), select the three-dot menu, then choose Advanced options. Select Repair first. Reopen Outlook and check whether the issue is fixed. If it is not, return to the same page and choose Reset. Reset clears the app’s local data and may require you to sign in and set up the app again. Your server-hosted mailbox is not meant to be erased, but confirm that important items are synced before resetting.
Clear Outlook site data in a browser. If the problem occurs in Outlook on the web, try a private or InPrivate window first. If Outlook works there, clear site data for Outlook in your usual browser rather than wiping all browsing data. In Microsoft Edge, go to Settings > Cookies and site permissions > Manage and delete cookies and site data > See all cookies and site data, search for Outlook or Microsoft 365 site entries, and remove the relevant data. In Chrome, go to Settings > Privacy and security > Third-party cookies > See all site data and permissions, search for the Outlook site, and delete its stored data. Close and reopen the browser, then sign in again. The labels may shift slightly between browser versions.
Refresh a folder in Outlook for Mac. In versions that provide the command, Control-click or right-click the affected folder in the folder list, choose Properties, and select Empty Cache. Outlook will fetch that folder’s contents again from the server. If Empty Cache is not shown, do not delete files from the Mac’s Library folders by guesswork. Confirm that the mailbox is synced, then use the account-management options in Outlook’s settings to remove and add the account again only if necessary. Removing an account can discard unsynced local items, so check for local-only folders and drafts first.
Check the result before cleaning up. Search for a recent message, open a few folders, and confirm that sending and receiving work. For a renamed OST, keep the old file until synchronization is complete and you have verified that Outlook contains the expected mail. Once you are confident that the new mailbox copy is complete, you can remove the old file to reclaim disk space.

Common Mistakes

Deleting the wrong data file. An OST for a server-based Microsoft 365 mailbox is normally a re-creatable offline copy. A PST may hold mail that is not stored online. Check the file extension and account before changing anything, and do not remove a PST just because it is listed beside an OST.

Assuming cache means the same thing in every Outlook. Clearing a folder’s offline items, rebuilding an OST, resetting the new Windows app, and clearing browser site data are different operations. Use the one that matches your app and symptom; resetting a whole app is unnecessary if only one folder needs refreshing.

Interrupting the first sync. Closing Outlook or disconnecting Wi-Fi while it is rebuilding can leave the mailbox apparently incomplete. Allow time for the download, particularly for large mailboxes, and check multiple folders before deciding that data is missing.

Clearing all browser data or removing an account without checking local items. A broad browser cleanup can sign you out of other sites and remove preferences. Removing an account or resetting an app can also remove local settings or unsynced content. Prefer targeted site data and verify synchronization first.

Tips

Make sure Outlook is online and not in Work Offline mode before judging whether mail has returned. A cache rebuild cannot download anything without a connection.
If only one folder is affected, refresh that folder before rebuilding an entire mailbox cache. This reduces the amount of data Outlook must download.
Keep Outlook updated. In classic Outlook, check File > Office Account > Update Options > Update Now, when Update Options is available. App and menu labels can differ by Microsoft 365 installation.
When using Outlook on the web, test in a private window before clearing data. If the issue disappears there, an extension or stored browser data may be involved; disable extensions one at a time or clear only the Outlook site’s data.
For a work or school mailbox, ask your Microsoft 365 administrator before changing account setup if the device is managed or subject to organizational policies.

Troubleshooting

Outlook does not recreate the OST: confirm that you renamed an OST belonging to the correct Microsoft 365 account, not a PST or another account’s file. Reopen Outlook while connected to the internet and wait. If it still fails, use File > Account Settings > Account Settings to confirm the account is present and connected. An administrator may need to check account access or a service issue.

Folders or messages appear to be missing: do not delete the renamed OST or empty the Recycle Bin. Check Outlook on the web to see whether the content exists on the server, then confirm the correct account and folder are selected in the desktop app. If a message existed only in a local data file, it may not be recoverable from the server; preserve the old file and contact your IT support team.

Synchronization is taking a long time: keep the computer awake, check the internet connection, and allow Outlook to finish. Large mailboxes and shared folders take longer to download. If progress appears stalled, restart Outlook once, check Microsoft 365 service status if available to you, and try again rather than repeatedly renaming files.

Reset or site-data clearing did not help: sign in again and test Outlook in another browser or app. If the issue affects multiple devices, it is less likely to be a single device’s cache and may involve the account, mailbox, network, or Microsoft 365 service. Record the exact error and contact your administrator or Microsoft support.

Frequently Asked Questions

Does clearing the Outlook cache delete my emails?

For a Microsoft 365 mailbox, mail is normally stored on the server and downloaded to Outlook as a local copy. Clearing or rebuilding that copy should not delete server mail. However, unsent drafts, local-only folders, or items in a PST may not be on the server, so verify those before resetting or removing files.

How often should I clear the Outlook 365 cache?

There is no routine schedule. Clear or rebuild cached data only when you have a specific problem, such as a folder that will not update or an app behaving incorrectly. Outlook uses its cache to make mail available and responsive, so clearing it without a reason can cause a long re-download.

Will clearing the cache make Outlook faster?

It can help when a local cache is corrupted or stuck, but it is not a general speed booster. After a rebuild, Outlook may be slower temporarily while it downloads messages again. If performance remains poor, check mailbox size, connection quality, add-ins, and whether the problem occurs in Outlook on the web too.

What is the difference between an OST and a PST?

An OST is generally an offline copy of a mailbox connected to a server, such as Microsoft 365 or Exchange, and Outlook can usually recreate it. A PST is a separate data file that can contain locally stored mail, calendar items, or archives. Treat PST files as potentially important and do not delete them as a cache-cleaning step.

Will clearing browser data sign me out of Outlook?

It can. Removing Outlook or Microsoft 365 site data may clear cookies and sign-in information for that site. Have your password and any required multi-factor authentication method available, and clear only the relevant site data if possible.

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