How to Clear Cache in the Outlook Desktop App Safely

To clear Outlook’s local mailbox cache in classic Outlook for Windows, close Outlook, rename the account’s offline data file (.ost), then reopen Outlook and let it rebuild. For new Outlook, use Windows’ app repair or reset options instead. First confirm your mailbox has synced: cache removal can discard changes that exist only on this device, and it is not the right fix for every Outlook account or every kind of stored data.

“Outlook cache” can mean different things. The offline mailbox cache is a local copy of email and calendar data; the Auto-Complete list stores recipient suggestions. Clearing one does not clear the other. The steps below distinguish the two and explain what to do if you use new Outlook or a Mac.

What You’ll Need

A Windows computer with Outlook installed, and access to the Windows account that runs it.
A few minutes when you can leave Outlook open to download mailbox data again after a rebuild.
Your work or personal account password, in case Outlook asks you to sign in again.
A confirmed internet connection and confidence that any recent messages in Outbox, drafts, or other folders have synced.

You do not need a separate cache-cleaning program. Avoid third-party cleaners and registry changes: they are unnecessary for the steps in this guide and can remove data or settings you did not intend to change.

Step-by-Step

Check which Outlook you have. Classic Outlook for Windows has a File tab and the full desktop ribbon. New Outlook usually has a Settings gear and no classic File tab. If you are unsure, open Outlook’s Start menu entry or app title and check whether it says “(classic)” or “(new).” The local-file steps below apply to classic Outlook, not the new app.
Make sure your mailbox is up to date. In classic Outlook, check the status bar at the bottom of the window for a connection or synchronization message. Open Outbox and Drafts and confirm that anything you need is present in the mailbox online as well. If you can, sign in to your organization’s webmail or Outlook.com and check for recent messages there. Do not proceed while Outlook is offline, still sending, or showing unsynced changes. A local-only item may not be recoverable after its cache is removed.
Find the offline file in classic Outlook. In Outlook, select File > Account Settings > Account Settings. On the Data Files tab, find the data file associated with the account and note its location. Select it and choose Open File Location if that button is available. The file you may rebuild is an .ost file. If you see a .pst file instead, stop: a PST can contain the only copy of locally stored mail, and the OST instructions are not a safe substitute for backing it up.
Close Outlook completely. Exit the app, then wait briefly. If Windows says the file is in use, open Task Manager with Ctrl+Shift+Esc, look for Outlook, and end the Outlook task only after confirming the window is closed. Do not interrupt Outlook while it is sending or synchronizing.
Rename the OST rather than deleting it. In File Explorer, locate the .ost file you identified. Right-click it, choose Rename, and add something like .old to the end of its name. If Windows hides file extensions, take care not to change the file into a different type accidentally. Renaming leaves a temporary fallback copy; do not rename or move a PST file.
Reopen classic Outlook and allow the cache to rebuild. Outlook should create a fresh OST and start syncing the mailbox from the server. Keep the computer online and Outlook open. The time required depends on mailbox size, connection speed, and any download-period setting. Mail may appear gradually, and search or calendar results can be incomplete until synchronization finishes.
Check that the rebuilt mailbox is usable. Confirm that recent messages, folders, and calendar items appear, and send a small test message if appropriate. If everything is present and Outlook works normally, you can later remove the renamed file to free disk space. Keep it until you are confident the new cache is complete; do not open or import it as though it were a backup mailbox.
Clear recipient suggestions only if that is what you meant. In classic Outlook, select File > Options > Mail. Under Send messages, choose Empty Auto-Complete List, then confirm. This removes suggested recipient addresses as you type; it does not delete email, contacts, or the offline mailbox cache. New Outlook has separate settings that can vary by build, so do not follow the classic menu path if those options are absent.
For new Outlook on Windows, repair or reset the app. Close Outlook. Open Windows Settings > Apps > Installed apps, find Outlook (new), select the three-dot menu, then choose Advanced options if shown. Try Repair first. If that does not help, choose Reset, then open Outlook and sign in again. Reset can remove local app settings and data, so verify that important messages and drafts have synced to the mailbox before using it.

On a Mac: Windows OST paths and Windows app-reset menus do not apply. Outlook for Mac manages account data differently. If you need to force a cloud mailbox to download again, first confirm everything is available in webmail, then use Outlook’s account settings to remove and re-add the account only if you understand the effect on local-only data. Menu labels can differ between classic and new Outlook for Mac; do not delete files from Library folders as a first step.

Common Mistakes

Deleting a PST as if it were disposable cache. An OST for a supported server mailbox is generally a rebuildable local copy. A PST may contain archived or locally stored messages that are not on the server. Identify the extension and account before changing any file.
Closing Outlook before synchronization finishes. An item visible on this computer may still be waiting to upload. Check Outbox, Drafts, and the same mailbox in a browser before rebuilding or resetting.
Expecting cache removal to fix every issue. A bad password, service outage, full mailbox, add-in conflict, or poor connection will not be corrected by replacing a local file. Diagnose those separately.
Removing more than one file or account at once. Changing several data files makes it difficult to tell which action helped and increases the chance of losing locally stored information. Work on only the identified account file.
Assuming Auto-Complete is the address book. Clearing recipient suggestions does not remove saved contacts, but it does mean Outlook may stop suggesting addresses you previously used. The suggestions can be rebuilt as you send mail.

Tips

Use a rebuild when Outlook shows stale or inconsistent server-backed mail, repeated synchronization errors, or a damaged offline cache—not as routine maintenance. A healthy cache does not need regular deletion.
Keep the renamed OST until the new one has finished syncing and you have checked important folders. If an item is missing, stop and investigate before deleting the old file.
After rebuilding, check classic Outlook’s account sync settings if older mail is not appearing. Some Exchange accounts download only a selected period of mail to the computer; older items may still be available online.
If your organization manages Outlook, ask its support team before changing account data. Company policies, shared mailboxes, or retention settings can affect what should be stored locally.
Prefer Repair over Reset for new Outlook when offered. Repair is the less disruptive first attempt; Reset is more likely to require signing in and setting up preferences again.

Troubleshooting

The OST will not rename because it is in use. Confirm Outlook is fully closed, including any background Outlook process in Task Manager. If a phone or another computer is syncing the mailbox, that does not normally lock the Windows file, but another Outlook process on this PC might. Restart Windows and try again before changing permissions or deleting files.

Outlook opens but does not create a new cache. Check that you renamed the OST for the correct account and that Outlook can connect to the internet and sign in. If the account is not a Microsoft Exchange, Microsoft 365, or Outlook.com mailbox, the file procedure may not apply. Restore the original filename after closing Outlook if needed, then contact your administrator or use Outlook’s account repair options rather than guessing at another file.

Some older messages are missing after the rebuild. Leave Outlook online while it completes synchronization, and check the mailbox in webmail. In classic Outlook, review the account’s offline mail download setting; a limited download window can make older messages available only when connected or through webmail. Do not delete the renamed OST until you understand whether the messages are on the server.

New Outlook still has the same problem after Repair or Reset. Check Microsoft 365 service status or ask your organization whether there is an outage. Install available Windows and Outlook updates, then sign in through webmail to distinguish an app problem from a mailbox or account problem. If webmail also fails, resetting the desktop app is unlikely to solve it.

Outlook for Mac is still out of sync. Confirm the account works in a browser, restart Outlook, and check for updates. If you are considering removing and re-adding the account, first verify that any local-only folders or unsent items are preserved. For a work account, consult your administrator before removing it.

Frequently Asked Questions

Will clearing Outlook’s cache delete my emails?
Rebuilding an OST for a server-backed account should download mailbox data again, but unsynced or local-only changes may be lost. Check webmail and protect important local items first. Do not delete a PST as a cache-cleaning step.

How long does Outlook take to rebuild the cache?
It depends on mailbox size, the amount of history selected for offline use, and your internet connection. Keep Outlook open and online; wait until folders and recent items finish appearing before judging whether the rebuild worked.

Does clearing the cache remove my Outlook password?
Renaming an OST normally does not itself remove saved Windows credentials, though Outlook may ask you to sign in again. Resetting new Outlook can clear app settings or sign-in state, so have your account credentials available.

Can I clear only Outlook’s autocomplete cache?
Yes. In classic Outlook for Windows, use File > Options > Mail > Empty Auto-Complete List. That clears address suggestions, not the mailbox cache or saved contacts.

Can I use these steps in Outlook on the web?
No. Outlook on the web runs in a browser and does not use the desktop OST file. If the web version behaves incorrectly, try the browser’s site-data or cache controls, or test in a private window; those actions are separate from clearing desktop Outlook’s cache.

Is it safe to clear the cache regularly?
Usually there is no need. A rebuild is a troubleshooting measure for a specific sync or data-file problem, not routine upkeep. If Outlook is working normally, leave its cache in place.

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