How to Clear the Offline Cache in Outlook

To clear Outlook’s offline cache in classic Outlook for Windows, close Outlook, find the account’s .ost file, and rename it; when Outlook starts again, it creates a fresh cache and downloads the account’s mail. Before you begin, make sure recent changes have synced and that you know your account password. These steps are for classic Outlook, not the newer Outlook for Windows app.

What You’ll Need

A Windows computer with classic Outlook installed and access to the account you want to refresh.
A working internet connection and the account password, plus any multi-factor authentication method required to sign in.
Enough free disk space and time for Outlook to rebuild the cache. A large mailbox may take a while to download.
Permission to access the Windows user profile folder where Outlook stores its data files.

Do not proceed by deleting files if you are unsure whether the account is a server-synced account or a POP account that keeps mail only on the computer. Read the cautions in the steps first.

Step-by-Step

Identify which Outlook you are using. The directions below apply to classic Outlook for Windows, which has a File tab in the ribbon. If your app is labeled Outlook (new) and does not have that classic File tab, skip to the section below on new Outlook. The two apps store offline data differently, so the classic .ost instructions do not apply to new Outlook.
Confirm the mailbox is online and synced. Open Outlook and check the status bar at the bottom of the window. It should show that the folders are up to date or that Outlook is connected. If it says Working Offline, select the Send / Receive tab and make sure Work Offline is not enabled. Allow outgoing messages to send, and check that recent incoming and sent messages appear. If Outlook cannot connect, fix the connection first; a cache rebuild cannot retrieve data while the account is unreachable.
Check the account type and protect local-only data. In Outlook, go to File > Account Settings > Account Settings, then select the account on the Email tab. Microsoft 365, Exchange, Outlook.com, and most IMAP mailboxes normally store their mail on a server and use an .ost offline cache. POP accounts commonly store messages in a .pst file instead. Do not delete a .pst file: it may be the only copy of messages, contacts, or calendar items. If you cannot tell which file belongs to the account, stop and ask your administrator or support person before removing anything.
Record any items that exist only on this computer. OST files are intended to be replaceable copies of server data, but changes that have not synced, local-only folders, or items in folders marked This computer only may not return after a rebuild. Check the Outbox, Drafts, and any custom local folders. If you use an Exchange account and have unsynced changes, connect Outlook and let it finish syncing. If you need to preserve local data, export or copy it to a separate PST using Outlook’s export options before proceeding. Do not treat the OST as a backup.
Locate the OST file from Outlook. In the Account Settings window, open the Data Files tab. Select the data file associated with the account and choose Open File Location if that button is available. The folder opens in File Explorer. Make a note of the file name and folder, then close the Account Settings windows. If the button is unavailable, use the file path shown in the list or ask your administrator; do not guess which file to remove.
Exit Outlook completely. Use File > Exit, or close the Outlook window and wait for it to shut down. Check the Windows taskbar notification area and make sure Outlook is not still running. If the file is in use, restart Windows and do not open Outlook again until you are ready to rename the file. Renaming an OST is safer than permanently deleting it because it leaves a temporary fallback copy.
Rename the OST file. In File Explorer, find the specific .ost file you noted. Right-click it, select Rename, and add a suffix such as .old to the existing name, for example account.ost.old. If Windows hides file extensions, avoid adding a second .ost by accident; the key is to make the original file name different so Outlook cannot reuse it. Do not rename or delete a .pst file, and do not change other Outlook data files in the folder.
Start classic Outlook and sign in if prompted. Outlook should detect that its offline cache is missing and create a replacement. If prompted, enter your account password and complete multi-factor authentication. Keep the computer connected to the internet and leave Outlook open while it downloads messages and folders. You may be able to work with newer content first, while older mail continues to populate.
Check that the new cache is rebuilding correctly. Watch Outlook’s status bar for synchronization messages. Open the Inbox, Sent Items, Calendar, and any important folders, and confirm that recent items appear. Send a small test message if appropriate. A large mailbox, slow connection, or server throttling can make the initial synchronization take hours; avoid repeatedly closing Outlook or renaming newly created files during this process.
Remove the old file only after confirming the result. Once the account works and the expected server-backed content is present, you can delete the renamed .old file to recover disk space. Keep it temporarily if you are uncertain or need help diagnosing the issue. It is not a substitute for a proper backup, and Outlook may not be able to use it after the new cache has been created.

For new Outlook for Windows: It does not use the classic Outlook .ost workflow described above. If the app is malfunctioning, close it and open Windows Settings > Apps > Installed apps (called Apps & features in some Windows versions). Find Outlook (new), select the three-dot menu, choose Advanced options, and try Repair first if it is available. If that does not help, choose Reset. Resetting can remove app data and sign-in state, so be ready to sign in again and let mail resynchronize. Confirm important mail is present in Outlook on the web before resetting.

Common Mistakes

Deleting a PST instead of an OST. A PST can contain the only copy of POP mail or archived items. Confirm the extension and account association before changing a file.
Removing the cache before syncing. Messages in the Outbox or changes that have not reached the server may be lost. Check Outlook’s connection and synchronization status first.
Deleting the file while Outlook is open. Outlook may still be using it, or it may recreate the file immediately. Exit the app fully before renaming the cache.
Expecting the download to finish instantly. Clearing the cache does not make the mailbox smaller or speed up the server. A large mailbox must be downloaded again, and search results may be incomplete while indexing catches up.
Following classic Outlook directions in the new app. The apps have different storage and settings. Use the app’s Repair or Reset options for new Outlook rather than looking for an OST that it does not use in the same way.
Confusing cache clearing with deleting mail. Removing a server-synced OST does not normally delete the server mailbox, but deleting messages while online does. Do not use the delete key on folders or mail as a cache-clearing shortcut.

Tips

Use a rename rather than permanent deletion at first. It gives you time to confirm that the fresh cache works before removing the old file.
Do this when you can leave the computer powered on and connected to a stable network. Keep the laptop plugged in if the mailbox is large.
If you only need to make Outlook download newer mail, try Send / Receive > Update Folder or Send / Receive All Folders before rebuilding the entire cache.
For Microsoft 365 or Exchange accounts, an administrator may set a download period for cached mail. If older messages are missing after the rebuild, check the account’s cached mail range rather than assuming the messages were erased.
Keep the renamed file only as a short-term precaution. It can consume substantial disk space, and it is not a supported long-term archive or backup.
On a Mac, there is no Windows OST file to rename. If Outlook for Mac has a sync problem, first check the connection and account status in Outlook. Removing and adding an account is a more disruptive step; confirm server copies and credentials first, and consult your organization’s support team for managed accounts.

Troubleshooting

Outlook says the file is in use. Close Outlook and wait a minute, then check Task Manager for any remaining Outlook process. If it remains locked, restart Windows and rename the file before opening Outlook. Do not force-delete the file while a process is using it.

Outlook does not create a new cache or asks for a profile. Check that you renamed the correct OST, not a PST, and that you are opening the same Windows profile and Outlook profile as before. If Outlook displays a sign-in prompt, complete it. If an error persists, use File > Account Settings > Account Settings to review the account, or contact your administrator before creating a new profile.

Mail is missing after the rebuild. First allow synchronization to finish and confirm Outlook is connected. Search Outlook on the web or your organization’s webmail for the missing message. If it exists online, the issue is likely the download range, folder synchronization, or a filter. If it is absent online and was stored only in a local folder or an unsynced change, the cache rebuild may not restore it; preserve the renamed file and seek help rather than deleting it.

The sync is stuck or very slow. Check the internet connection, available disk space, and Outlook’s status bar. Pause large downloads or VPN changes only if permitted by your organization. Try updating one folder and allow time for the rest. If the status shows an error, record its exact wording and contact your Microsoft 365 administrator or support team; repeated cache resets can make recovery harder.

New Outlook still has a problem after Reset. Reopen the app, sign in, and wait for the account to synchronize. If the issue continues, install pending Windows or Outlook updates, try Repair if you have not already, and report the error through your organization’s support channel. Do not apply classic Outlook registry edits or delete random folders from AppData.

Frequently Asked Questions

Will clearing the offline cache delete my emails?
For a correctly configured Microsoft 365, Exchange, Outlook.com, or IMAP account, the OST is a local copy and Outlook downloads server-stored mail again. Local-only items and unsynced changes may not return. A PST can hold mail locally, so never delete it as part of this process.

How long does Outlook take to rebuild the cache?
It depends on mailbox size, connection speed, server response, and the cached mail range. A small mailbox may finish quickly; a large one can take hours. Keep Outlook online and open, and check the status bar rather than restarting the process repeatedly.

Can I clear only one folder’s cache?
In classic Outlook, updating a folder through Send / Receive > Update Folder can prompt a refresh without rebuilding every folder. A complete OST rename rebuilds the account’s local cache, not just one folder. For persistent folder-specific sync errors, contact your administrator before using advanced repair steps.

Is it safe to clear the cache while Outlook is offline?
It is safer to wait until Outlook is connected and has finished sending and syncing. If you clear the cache while offline, changes that exist only on that computer may be lost, and Outlook cannot download a replacement until it reconnects.

Does this guide work for Outlook on Mac?
No. Mac Outlook does not use the Windows OST file path or File > Account Settings workflow in this guide. Check the account’s sync status in Outlook for Mac first. If you need to remove and add the account, confirm that messages are available online and that you have the correct sign-in details.

Should I clear the cache regularly?
No routine clearing is needed. Rebuild it when troubleshooting a persistent synchronization or cache problem, or when support recommends it. Frequent resets make Outlook download the same data repeatedly and can temporarily reduce access to older offline mail.

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.