How to Clear Outlook Cache: A Complete Guide for Desktop and Web

• 21 June 2026
• by: Chloe Martin
• in: Server Performance
• Tags: business email, email troubleshooting, ost file, outlook
• note: no comments

Microsoft Outlook keeps several layers of cached data to make your inbox feel fast: a list of recently typed recipients, a local copy of your entire mailbox, and a folder full of opened attachments. Most of the time these caches work silently in the background. But when an old email address keeps autocompleting, a folder refuses to sync, or your profile starts behaving strangely, clearing the relevant Outlook cache is one of the most reliable troubleshooting steps available.

This guide explains what each Outlook cache actually stores, how to clear it on the desktop client and in the new Outlook / Outlook on the web, and—critically—when clearing the cache is safe versus when it can cost you data.

Key Takeaways
• Outlook maintains multiple separate caches: the autocomplete (nickname) cache, the offline OST file, and the temporary attachment cache.
• Clearing the autocomplete list fixes stale or wrong email addresses that keep suggesting themselves.
• Rebuilding the OST file is safe for Exchange and IMAP accounts because the data re-downloads from the server.
• For POP3 accounts, deleting local data can be permanent—back up first.
• The new Outlook and Outlook on the web cache differently; you clear them through the browser or app settings rather than file deletion.

Why Should You Clear the Outlook Cache?

Cached data is meant to help, but it can become a liability. Clearing the cache addresses several common problems:

• Stale autocomplete addresses. Outlook keeps suggesting an old, misspelled, or decommissioned address, increasing the risk of bounced or misdirected mail.
• Sync issues. Folders show different contents than the server, messages appear stuck in the Outbox, or read/unread states are inconsistent across devices.
• Profile corruption. A damaged local cache file can cause crashes, freezes, or errors when opening items.
• Wasted disk space. A large offline cache and an accumulated attachment temp folder can consume gigabytes over time.
• General troubleshooting. Many Microsoft support steps begin with “clear the cache” because it forces Outlook to rebuild a clean local copy from the authoritative server.

The key insight is that, for server-based accounts, the server is the source of truth. The local cache is just a convenience copy—so clearing it rarely means losing anything.

What Are the Different Types of Outlook Cache?

Outlook does not have one single “cache” button. Instead, it manages distinct stores, each cleared a different way. The table below summarizes them.

Cache Type	What It Stores	Where It Lives	How to Clear It
Autocomplete / nickname cache	Recently used recipient names and addresses (the “type-ahead” list)	`RoamCache` folder / mailbox stream	Empty the Auto-Complete List in Outlook options, or delete individual suggestions
Offline cache (OST)	A full local copy of your Exchange/IMAP mailbox for offline access	`.ost` file in the Outlook data folder	Close Outlook, rename or delete the `.ost`; it rebuilds from the server
Temporary attachment cache	Copies of attachments you have opened directly from messages	`SecureTemp` / `Content.Outlook` folder	Empty the SecureTemp folder via its registry-defined path
New Outlook / web cache	Rendered UI, message previews, and offline data in the app or browser	App data store / browser site data	Clear via app settings or the browser’s site-data/cache controls

Understanding which cache is causing your problem saves time—you only clear what you need.

How Do You Clear the Outlook Autocomplete Cache?

The autocomplete cache (also called the nickname cache) is the list of suggestions that appears as you type in the To, Cc, or Bcc fields. It is the most common cache people want to clear, usually because a wrong address keeps appearing.

To remove a single bad suggestion:

1. Start typing the recipient’s name or address in a new email.
2. When the incorrect entry appears in the dropdown, hover over it (or use the arrow keys to highlight it).
3. Click the X next to the entry, or press Delete.

To empty the entire autocomplete list (classic desktop Outlook):

1. Go to File → Options → Mail.
2. Scroll to the Send messages section.
3. Click Empty Auto-Complete List.
4. Confirm when prompted.

This clears every saved suggestion at once. Outlook will rebuild the list naturally as you send new messages. Note that the autocomplete list is separate from your Contacts—emptying it does not delete any contact records.

How Do You Clear the Offline Cache (OST File)?

The OST file is a local, offline copy of a mailbox. Outlook uses it in Cached Exchange Mode and for IMAP accounts so you can read and search mail without a live connection. When it becomes bloated or corrupted, rebuilding it often resolves stubborn sync errors.

To rebuild the OST file:

1. Close Outlook completely. Confirm it is not running in the background (check the system tray).
2. Open the data file location. In classic Outlook, go to File → Account Settings → Account Settings → Data Files, select your account, and click Open File Location.
3. With Outlook closed, rename the `.ost` file (for example, add `.old` to the end) rather than deleting it outright—this gives you a fallback.
4. Reopen Outlook. It will create a fresh `.ost` and re-download your mailbox from the server.
5. Once you confirm everything synced correctly, you can delete the renamed `.old` file to reclaim space.

Renaming instead of deleting is a deliberate safety habit: if anything looks wrong after the rebuild, you can close Outlook and restore the original file.

Is It Safe to Delete the OST File? It Depends on Your Account Type

Here is the distinction that trips people up most often. Whether deleting the OST is safe depends entirely on how your account stores mail.

• Exchange / Microsoft 365 / IMAP accounts: The OST is purely a *cache*. Every message, folder, and setting also exists on the server. Deleting or renaming the OST simply forces a clean re-download. This is safe and is exactly what Microsoft’s own troubleshooting steps recommend.
• POP3 accounts: This is the trap. POP3 traditionally downloads mail to your device and may remove it from the server. In that scenario, your local store can be the *only* copy of older messages. POP accounts use a `.pst` file rather than an `.ost`, but the broader lesson holds: never delete local POP data without a verified backup, because there may be nothing on the server to restore from.

Before touching any local data file, identify your account type under File → Account Settings. If it says POP, export to a backup PST first (File → Open & Export → Import/Export → Export to a file). If it says Exchange, Microsoft 365, or IMAP, you can rebuild the cache with confidence.

How Do You Clear the Temporary Attachment Cache?

When you open an attachment directly from an email (rather than saving it first), Outlook stores a copy in a hidden SecureTemp folder—often labeled `Content.Outlook`. Over time this folder accumulates orphaned files and can grow large or even prevent you from opening attachments with the same filename.

To clear the attachment cache:

1. Close Outlook.
2. The folder path is defined by a registry value named OutlookSecureTempFolder. Because the path is long and randomized, the reliable way to find it is to look it up in the registry under the Outlook Security key, then paste that path into File Explorer.
3. Select the contents of the folder and delete them. The files here are disposable copies—your original attachments remain safe inside the original emails.
4. Reopen Outlook.

Because editing the registry carries some risk, only navigate to read the path value; you do not need to change any registry data to clear this cache.

How Do You Clear the Cache in New Outlook or Outlook on the Web?

The new Outlook for Windows and Outlook on the web are built on web technology, so they cache differently from the classic desktop client. There is no OST file to rename.

For Outlook on the web (browser):

1. Sign out of Outlook on the web.
2. In your browser, clear cached images and files and cookies and site data for the Outlook/Office domain (Settings → Privacy → site data).
3. Alternatively, open Outlook in a private/incognito window to confirm whether a caching issue is the cause.
4. Sign back in; the web app rebuilds its local state.

For the new Outlook app:

1. Open Settings within the app.
2. Look under General → Storage (or the account section) for options to clear locally stored data, then resync the account.
3. If problems persist, removing and re-adding the account forces a complete fresh download from the server.

Because both rely on the server-side mailbox, clearing this cached layer is low-risk for Exchange and Microsoft 365 accounts.

How Reliable Server-Side Email Makes Cache Clearing Painless

Everything above hinges on one assumption: your mail server holds a complete, current copy of your mailbox. When that is true, clearing any Outlook cache is a safe, reversible action—the client just re-downloads what it needs.

That reliability is exactly what professional business email hosting is built to deliver. DarazHost provides business email hosting that works cleanly with Outlook over standard IMAP and SMTP, so your messages live safely on the server and stay in sync across desktop, the new Outlook, mobile, and webmail. Because mail is stored server-side, rebuilding an OST or clearing the autocomplete list never puts your correspondence at risk—it simply refreshes the local copy.

If you are setting up Outlook with your own domain email and want dependable sync, generous storage, and webmail access as a fallback, DarazHost’s email hosting plans are designed for it. Our 24/7 support team can walk you through configuring Outlook with the correct IMAP/SMTP settings for your domain, so is straightforward from day one.

Best Practices When Clearing Any Outlook Cache

• Confirm your account type first. IMAP/Exchange caches rebuild safely; POP data may be local-only.
• Rename rather than delete data files so you can roll back if needed.
• Back up POP3 mailboxes to a PST before touching local files.
• Close Outlook fully before deleting or renaming any cache file.
• Clear only the cache that matches your symptom—autocomplete for wrong addresses, OST for sync errors, SecureTemp for attachment issues.

Frequently Asked Questions

Will clearing the Outlook cache delete my emails? For Exchange, Microsoft 365, and IMAP accounts, no—your mail lives on the server and re-downloads automatically. For POP3 accounts, it can, because mail may exist only on your device. Always verify your account type and back up POP data first.

Why does Outlook keep suggesting an old or wrong email address? That address is stored in the autocomplete (nickname) cache, which is separate from your Contacts. Delete the single suggestion by clicking the X beside it, or empty the whole list via File → Options → Mail → Empty Auto-Complete List.

Where is the Outlook OST file located? Typically in your user profile’s Outlook data folder. The reliable way to find it is File → Account Settings → Account Settings → Data Files → Open File Location. Always close Outlook before renaming or deleting it.

How do I clear the cache in the new Outlook or Outlook on the web? There is no OST file to rename. Clear cached files and site data for the Outlook domain in your browser, or use the app’s storage settings to clear local data and resync. Removing and re-adding the account forces a full refresh.

How often should I clear the Outlook cache? There is no need to clear it on a schedule. Treat cache clearing as a targeted troubleshooting step—do it when you see stale addresses, sync errors, attachment problems, or unexplained crashes, not as routine maintenance.

Conclusion

Outlook’s caching layers exist to make email faster and available offline, but they occasionally need a reset. Match the fix to the symptom: empty the autocomplete list for stubborn wrong addresses, rebuild the OST for sync trouble, clear SecureTemp for attachment glitches, and use browser or app settings for new Outlook and the web. Above all, remember the account-type rule—IMAP and Exchange caches rebuild safely from the server, while POP3 data can be irreplaceable. With reliable server-side email hosting behind your mailbox, clearing the cache becomes a routine, low-stress part of keeping Outlook running smoothly.