How to Clear Cache in MS Outlook Without Losing Mail

To clear Outlook’s local mailbox cache in classic Outlook for Windows, close Outlook, rename its Exchange or Microsoft 365 .ost file, then reopen Outlook and let it sync a fresh copy. This does not normally delete mail stored on the server, but “cache” can also mean autocomplete suggestions or browser data, so first identify which version and data you want to clear. Do not delete a .pst file; it may contain the only copy of locally stored mail.

What You’ll Need

A few minutes to identify your Outlook version and account type. The steps for classic Outlook, new Outlook, Outlook for Mac, and Outlook on the web are different.
For the classic Windows mailbox-cache method, access to the Windows account that runs Outlook and enough disk space for the mailbox to sync again.
A working internet connection and your sign-in details or access to your organization’s sign-in method, in case Outlook asks you to authenticate again.
If you have mail, calendar entries, or contacts stored only on this computer, an appropriate backup before changing local Outlook data.

Step-by-Step

Choose the kind of cache you mean. If Outlook search, folders, or messages appear stale, you may need to rebuild the local mailbox cache. If you want to remove suggested recipient names, clear the Auto-Complete list instead. If Outlook runs in a browser, clear site data or cached files in that browser. These are separate actions: clearing autocomplete does not rebuild your mailbox, and clearing browser data does not erase the mailbox stored by Microsoft 365 or Exchange.
Check which Windows Outlook app you have. Classic Outlook has a File tab at the top left and an Account Settings command. New Outlook has a more web-like interface and typically does not have the same File tab. Follow the matching Windows instructions below; do not use classic Outlook’s OST-file procedure on new Outlook.
For classic Outlook, confirm that the account uses an OST file. Open Outlook and select File > Account Settings > Account Settings. On the Email tab, check the account type. Exchange and many Microsoft 365 work or school accounts use an OST cache. A POP account may use a PST data file instead. If the account is POP, or you are unsure whether a file contains locally stored information, stop before changing files. Do not rename or delete a PST as a cache-clearing shortcut.
Protect anything that may be stored only on the computer. Confirm that important messages, calendar entries, and contacts are visible in Outlook on the web or another device, where available. Check the Outbox for unsent messages and allow pending changes to sync. An OST is generally a synchronized copy of server data, but local-only items or unsynchronized changes may not be recoverable from the server. If you cannot confirm that needed data is synced, ask your IT administrator for help before rebuilding the cache.
Close classic Outlook completely. Save any open message or appointment, then use File > Exit if available. Wait a few seconds. If Outlook remains open in the background, the OST may be locked. You can check the Windows taskbar or Task Manager to confirm Outlook has quit; do not force-close it while it is sending or saving work.
Open the Outlook data folder. Press Windows key + R, enter %LOCALAPPDATA%\Microsoft\Outlook, and press Enter. This opens the usual local Outlook data folder for the current Windows user. If that location does not contain the file you need, return to File > Account Settings > Account Settings before closing Outlook, select Data Files, and use the file location shown for the relevant account. Locations can differ when an organization has configured Outlook differently.
Rename the OST file, rather than deleting files indiscriminately. In File Explorer, look for an .ost file that belongs to the account you are repairing. If file extensions are hidden, use View > Show > File name extensions in Windows 11, or the equivalent View option in your Windows version. Right-click the relevant OST, choose Rename, and add a suffix such as .old to its existing name, keeping a note of the original name and location. Rename only the confirmed OST. Leave PST files and other Outlook files alone. If Windows says the file is in use, make sure Outlook has fully closed rather than trying to force the rename.
Restart Outlook and allow it to rebuild. Open classic Outlook. It should create a new OST and download the mailbox data from Exchange or Microsoft 365. Keep the computer online and Outlook open while folders and messages synchronize. A large mailbox, slow connection, or many shared folders can make this take a while. You may see fewer messages at first; wait for synchronization to finish before deciding that mail is missing. Once the account is working and the data looks right, you can remove the renamed backup later if you no longer need it.
If you only want to clear recipient suggestions, use the built-in control instead. In classic Outlook for Windows, select File > Options > Mail. Under Send messages, select Empty Auto-Complete List and confirm. This removes the stored suggestions Outlook offers as you type addresses; it does not remove contacts from your address book or delete messages. To remove just one suggestion while composing, start typing the address and select the X beside the unwanted suggestion when it appears.
For new Outlook on Windows, use the app’s reset option if the app itself is stuck. Close Outlook, open Windows Settings > Apps > Installed apps, find Outlook (new), select its three-dot menu, then choose Advanced options > Reset if that option is available. Windows may show a confirmation. Resetting clears local app data and may sign you out, so make sure you can sign in again. Your online mailbox normally remains on the server and can resynchronize after you sign in. This is an app reset, not the classic Outlook OST procedure.
For Outlook on the web, clear only the relevant browser data. First sign out of Outlook and close its tabs. In your browser’s privacy or site-data settings, remove cached files or site data for the Outlook or Microsoft 365 site you use, rather than clearing all browser data unless you intend to do so. The exact menu names vary by browser. Reopen the site, sign in, and check whether the issue is resolved. Clearing cookies can sign you out of other Microsoft services as well.

Mac note: Outlook for Mac does not use the Windows OST-file steps above, and there is no single equivalent cache-reset command that is appropriate for every Mac setup. Start by quitting and reopening Outlook and checking for updates. If a particular account still has syncing problems, verify that its contents are available on the server before removing and adding the account again. Do not delete folders from the Mac Library or remove a profile unless you understand whether it contains local-only data; workplace-managed Macs may require IT support.

Common Mistakes

Deleting a PST file. A PST can hold mail, contacts, or archives that are not stored on the server. It is not interchangeable with a synchronized OST. Do not delete or rename it just because its name includes Outlook.
Assuming every Outlook issue is a cache problem. A service outage, network problem, full mailbox, incorrect password, or damaged Outlook profile can look like stale data. Check Outlook on the web or Microsoft 365 service status, if available, before removing local data.
Interrupting the first sync. After a rebuild, the mailbox may appear incomplete until synchronization finishes. Keep Outlook online and allow time for folders and shared mailboxes to update before judging the result.
Clearing the wrong thing. Emptying Auto-Complete removes address suggestions, not the mailbox cache. Clearing browser files affects Outlook on the web, not the desktop app’s OST. Choose the action that matches the symptom.
Changing files while Outlook is open. Outlook may keep its data file locked or write changes while you are trying to rename it. Close the app fully first and let pending sends and sync operations finish.

Tips

Use the least disruptive fix first. Restart Outlook, check your connection, and see whether the affected message or folder is correct in Outlook on the web. If the web version is current but classic Outlook is not, a local cache rebuild is more likely to help.
Rename instead of immediately deleting. Keeping the old OST temporarily gives you a way to restore the file if you renamed the wrong one. Do not open or rely on that old file as a backup of server mail; it is a cache, not a dependable archive.
Plan for a large download. Rebuilding a mailbox can use bandwidth and may temporarily make search results incomplete. If you use a metered connection or have a very large mailbox, choose a time when the computer can stay connected.
Keep local folders distinct from synced folders. In classic Outlook, folders labeled On My Computer or content in a personal data file may not be part of the online mailbox. Back up or export data using an appropriate Outlook method before account or profile changes.
Use autocomplete clearing selectively. Empty the entire list only if you want all suggestions removed. If one incorrect or outdated address is the problem, remove that individual suggestion while composing instead.
Record what you changed. Note the original data-file name, account type, and any error message. This makes it easier to reverse a rename or explain the issue to your organization’s support team.

Troubleshooting

The OST file is not in the suggested folder. The location can vary by account and organization. In classic Outlook, check File > Account Settings > Account Settings > Data Files while Outlook is open and note the location for the account. Close Outlook before changing anything. If you do not see an OST or cannot identify the correct one, do not guess; contact your administrator.

Windows says the file is being used. Close Outlook and check that it has exited fully. If necessary, restart the computer and try again before opening Outlook. Avoid deleting files from Task Manager or ending processes while Outlook may still be sending or saving data.

Outlook does not recreate the OST or asks you to sign in. Check the internet connection, sign in with the account that owns the mailbox, and complete any multi-factor authentication prompt. If the account is disabled, the password has changed, or your organization requires a managed device, an administrator may need to restore access or configure the account.

Messages or folders are still missing after the rebuild. Wait for synchronization and check the account in Outlook on the web. If the items are missing there too, rebuilding the local cache will not restore them; check Deleted Items, retention policies, or ask your organization’s support team. If the items exist online but not in Outlook, check the folder list and sync status, then contact support if syncing does not complete.

Search is still incomplete. Search may not include newly downloaded messages until Outlook and Windows finish indexing. Leave the computer powered on and online, then test again. If only one folder or mailbox is affected, check its sync settings and whether it is shared. Do not repeatedly rebuild the cache, since each rebuild triggers another full synchronization.

The reset option is missing in new Outlook. Windows settings and app-management options can differ by Windows release and workplace policy. Update the app through the available Microsoft Store or organization-managed process, restart Windows, and try again. If the app is managed, ask IT before changing its data or reinstalling it.

Frequently Asked Questions

Will clearing Outlook’s cache delete my emails?
Rebuilding a classic Outlook OST normally downloads synced mail again from the Exchange or Microsoft 365 server. It does not protect local-only items or unsynchronized changes, so verify those first. Never delete a PST unless you have confirmed what it contains and have a backup.

How often should I clear the Outlook cache?
There is no routine schedule. Clear or rebuild it only when you have a specific problem, such as persistent stale data or a sync fault, and simpler checks have not helped. Frequent rebuilds waste time and bandwidth.

Does Empty Auto-Complete List delete my contacts?
No. It removes Outlook’s saved address suggestions. Contacts stored in your address book remain, and you can still select them from the address book when addressing a message.

Can I clear the cache without closing Outlook?
Do not rename or remove a desktop Outlook data file while Outlook is using it. Close classic Outlook fully first. The built-in Auto-Complete control can be used from Outlook settings, but close and reopen Outlook afterward if the change does not appear immediately.

Is clearing cache the same in Outlook for Mac?
No. The Windows OST procedure does not apply to Mac. Restart and update Outlook first; before removing an account or profile, confirm that needed content is synced or backed up. If the account is managed or contains local-only data, ask your administrator for the safest repair method.

What if Outlook on the web is also showing outdated mail?
A desktop cache rebuild is unlikely to fix a problem that also appears online. Check that you are in the correct account and folder, confirm your connection, and check for a Microsoft 365 service issue. For work or school mail, contact your administrator if the server copy remains wrong.

Try it: the Date Difference Calculator: days, months, years and weekdays between two dates, with the DATEDIF formulas.

More in How-to basics

Sources: Function behaviour and menu paths: Microsoft Support documentation for Excel. Method & sources

Spotted an error in this guide? Let us know.