Clear Email Cache in Outlook 365: Safe Steps

To clear the email cache in classic Outlook for Microsoft 365 on Windows, close Outlook, then rename the account’s offline Outlook Data File (.ost) in %LOCALAPPDATA%\Microsoft\Outlook; reopen Outlook and let it rebuild the file. First check that the account is Microsoft 365 or Exchange and that pending messages have sent—do not delete a .pst file or anything that may contain unsynced mail.

What Outlook’s email cache contains

Outlook uses the word cache to mean locally stored information that helps the app display mail and work when a connection is slow or unavailable. In classic Windows Outlook, Cached Exchange Mode stores a synchronized copy of mailbox data in an .ost file on the computer. Outlook reads much of that local copy rather than fetching every item from the server each time.

That cache is different from several other things people may want to clear:

Autocomplete suggestions: Names and addresses Outlook offers while you type in the To, Cc, or Bcc fields.
Browser cache: Temporary website files held by Edge, Chrome, Safari, or another browser when using Outlook on the web.
Mailbox content: Actual messages, calendar items, and contacts stored in the account. Clearing a cache is not the same as deleting these server-side items.
Outlook settings and profiles: Account configuration and preferences, which may require separate repair steps.

Identifying which one is causing the problem matters. Rebuilding an .ost may help with a stale or damaged offline mailbox, but it will not necessarily remove an incorrect recipient suggestion or fix a browser sign-in issue.

Choose the right steps for your Outlook version

Microsoft 365 can refer to different Outlook apps. Classic Outlook for Windows is the desktop program with a File tab and the full ribbon. New Outlook for Windows has a more web-like interface and does not use the same user-managed .ost workflow. Outlook on the web runs in a browser, while Outlook for Mac has its own account and local-data controls.

Before changing anything, check the app you have open. If you see File > Options, you are using classic Outlook for Windows. If there is no File tab and settings are opened from a gear icon, you may be in new Outlook or Outlook on the web. Use the matching section below rather than following instructions for a different app.

Rebuild the cache in classic Outlook for Windows

For a Microsoft 365 or Exchange account in classic Outlook, rebuilding its .ost is a common way to force a fresh local synchronization. The mailbox copy on the server is normally retained, but changes that exist only on the computer may not be. Take these precautions first:

Connect to the internet and allow Outlook to finish sending and receiving.
Check the Outbox for messages that have not been sent. Open and copy their contents if necessary.
If you use an account with locally stored folders or imported data, confirm whether it uses a .pst file. Do not remove that file as part of this process.
If the computer is managed by an employer, check with IT before changing account data files.

In Outlook, choose File > Account Settings > Account Settings.
On the Email tab, confirm which account is affected. On the Data Files tab, you can inspect the listed data files and their locations.
Close Outlook completely. Check Task Manager if necessary to make sure Outlook is no longer running.
Open File Explorer, enter %LOCALAPPDATA%\Microsoft\Outlook in the address bar, and locate the relevant .ost file. If the path or file is not obvious, use the Data Files tab to identify the data file rather than guessing.
Rename the .ost file—for example, add .old to its name—instead of permanently deleting it. Reopen Outlook and allow it to download and synchronize the mailbox again.

Mail may appear gradually while the rebuild runs. Large mailboxes, slow connections, and shared folders can make the initial synchronization take a while. Keep Outlook open and connected, and avoid repeatedly renaming files during the process. If everything is working and you have verified the needed data, you can remove the old renamed file later. If Outlook does not recreate the cache or reports an account error, stop and contact your administrator or Microsoft support rather than deleting other data files.

Clear autocomplete email suggestions

If the problem is an outdated address that appears as you type a recipient, rebuilding the mailbox cache is unnecessary. In classic Outlook for Windows, use File > Options > Mail. Under Send messages, select Empty Auto-Complete List and confirm. This clears the stored suggestion list, so Outlook will learn suggestions again from future use. It does not delete contacts from the address book or remove messages.

To remove just one suggestion, start typing the name or address in a recipient field. When the unwanted suggestion appears, select its remove control if shown; in many classic Outlook versions, you can highlight the suggestion with the arrow keys and press Delete. The exact control can differ between releases. In new Outlook and Outlook on the web, suggestion management is not identical to classic Outlook’s Options page, so use the remove option displayed beside the specific suggestion when available, or review the app’s settings for privacy and suggestions.

New Outlook for Windows: reset app data carefully

New Outlook does not expose the classic Outlook .ost file as a routine cache-clearing control. If it is misbehaving, first quit and reopen the app, check for updates, and confirm that the account can access mail in a browser. This helps distinguish a local app issue from a mailbox or service problem.

If you specifically need to reset the app, Windows may provide the option at Settings > Apps > Installed apps > Outlook (new) > Advanced options > Reset. The wording or availability can vary by Windows build and installation. Resetting can clear local app data and preferences, so make sure you know the account sign-in details and understand that settings may need to be configured again. Mail that is synchronized with Microsoft 365 should download again after you sign in, but do not assume that any unsynced or locally held information is backed up. If the Reset option is absent, do not try to substitute classic Outlook’s .ost deletion steps.

Outlook on the web and Outlook for Mac

For Outlook on the web, there is no Outlook desktop .ost file to delete. Refresh the page, sign out and back in if appropriate, or clear the browser’s cached files for troubleshooting. In the browser’s privacy or history settings, choose cached images and files rather than clearing all site data by default. Clearing cookies or site data can sign you out and remove site preferences, so make sure you can complete the sign-in process before doing that. If only one browser has the problem, try Outlook in another supported browser to help isolate it.

Outlook for Mac also does not use the Windows .ost procedure. Start with quitting and reopening Outlook and checking the account’s connection. In some Mac releases, an account-level reset or re-sync option is available from the account controls; its location and availability can vary. Use an option explicitly labeled Reset Account only when you understand that local account data may be downloaded again, and allow synchronization to finish. If the option is not present, consult Microsoft’s current instructions for your exact Outlook for Mac version or ask your organization’s IT team instead of deleting files from Library folders by guesswork.

When clearing the cache will not solve the problem

A cache reset is useful when the local copy is stale or corrupted, but many Outlook symptoms have another cause. If new mail is missing both in Outlook and in Outlook on the web, investigate account access, mailbox rules, filters, storage limits, or a service issue before rebuilding local data. If messages appear on the web but not in classic Outlook, check the connection status and synchronization state in the desktop app.

For a message stuck in the Outbox, inspect its attachment size, network connection, and recipient details. For repeated password prompts, check account authentication and any organization sign-in requirements; deleting an .ost is not a password fix. For a shared mailbox, confirm that you have permission and that the mailbox is configured correctly. If the problem began after an Office update, installing available updates or repairing Microsoft 365 from Windows settings may be more relevant than clearing data.

A practical diagnostic is to compare the same mailbox in Outlook and in a browser. If the item exists on the server but not in the desktop app, a local synchronization problem is more plausible. If the item is absent everywhere, look for server-side deletion, filtering, or account-level issues before changing local files.

Safety, backups, and what to expect

An .ost is generally a synchronized offline copy for an Exchange or Microsoft 365 mailbox, not the authoritative mailbox itself. However, that does not guarantee every item on the computer is safely represented online. Work performed offline, messages still waiting to send, or data in a separate local folder can require special care. That is why renaming the identified .ost is safer than deleting every Outlook file, and why confirming synchronization first is important.

A .pst is different: it can hold mail, archives, or imported folders that are not stored in the online mailbox. Never delete a .pst just because Outlook is slow or because its filename looks unfamiliar. If you are uncertain which file belongs to which account, stop and ask an administrator or support technician. After a cache rebuild, expect Outlook to use network bandwidth and disk space while it downloads data, and some older items may take longer to display.

Frequently Asked Questions

Will clearing the Outlook cache delete my emails?
Rebuilding an .ost for a synchronized Microsoft 365 or Exchange account normally makes Outlook download the mailbox again; it is not intended to delete server mail. Still, verify synchronization and protect anything that may exist only locally before proceeding.

How long does Outlook take to rebuild its cache?
There is no fixed time. Mailbox size, connection speed, shared folders, and device performance all affect the download. Keep Outlook connected and give it time to finish rather than interrupting the rebuild.

Can I delete the Outlook folder in AppData?
Do not delete the whole folder. It can contain multiple data files and other account information. Identify the correct .ost for the affected account and rename only that file, or ask IT for help if you cannot identify it safely.

Does clearing the cache remove autocomplete addresses?
Not necessarily. The autocomplete list is separate from the mailbox cache. In classic Outlook, use File > Options > Mail > Empty Auto-Complete List, or remove an individual suggestion as it appears.

Should I clear the cache if Outlook will not send email?
Not as the first step. Check the Outbox, connection, attachment size, and account sign-in. A cache rebuild may help a synchronization fault, but it will not correct every sending problem and could complicate recovery of unsent work.

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