How to Reduce Outlook OST File Size Without Losing Mail

How to reduce Outlook OST file size without losing mail comes down to two levers: compacting the file so Outlook reclaims unused space on disk, and narrowing the sync window so it caches less going forward. Both are safe operations on a properly configured Exchange or Microsoft 365 account. Neither deletes mail from the server.

An OST file is Outlook's local offline cache. In Cached Exchange Mode, Outlook downloads a copy of your mailbox to your hard drive so you can read and compose mail without an active connection. The file grows as mail arrives and does not automatically shrink when you delete messages. On an account with the default 12-month sync window, the OST can reach 20, 30, or even 50 gigabytes on a busy inbox.

This guide covers the verified steps for classic Outlook (versions 2016 through current Microsoft 365 Apps), the equivalent settings in New Outlook for Windows, what to do when the standard steps do not produce the expected result, and when it is safe to delete the OST file entirely and let Outlook rebuild it from the server.

Why the OST file keeps growing even after you delete mail#

Outlook stores the OST as a proprietary database file. When you delete a message, Outlook marks that space as available for reuse rather than immediately returning it to the operating system — the same behavior a relational database exhibits when you delete rows. The space on disk stays allocated. Over time, the unused portion accumulates inside the file. That accumulated unused space is what Compact Now reclaims: it rewrites the file, packs the live content together, and releases the rest back to the OS.

The second driver of OST growth is the sync window. Classic Outlook defaults to downloading the last 12 months of mail from the server. On an active inbox receiving dozens of messages per day, 12 months of content adds up quickly.

Attachments compound the problem: a 15 MB PDF counts at its full size in the local cache, just as it does on the server. Calendar invites with embedded files, email threads with inline images, and long Sent Items histories all contribute to the total.

The practical result is that OST size and server mailbox size can diverge. You can have a mailbox comfortably within quota on the server while the OST file is crowding a laptop SSD. Compacting addresses the white-space problem. Narrowing the sync window addresses how much new content Outlook keeps pulling in. Used together, they are more effective than either one alone.

Before you start#

Check two things before running any of the steps below. First, Compact Now only does visible work if Outlook estimates that more than 20 percent of the file is unused white space. If you have not deleted much mail recently, running Compact Now may produce almost no change in file size. That is expected behavior, not a malfunction. Delete items and empty the Deleted Items folder first, then compact.

Second, confirm your account type before considering a delete-and-rebuild. For Exchange and Microsoft 365 accounts, the OST is a synchronized cache and the authoritative copy of your mail lives on the server. Deleting the OST and relaunching Outlook causes Outlook to recreate it from the server — nothing is lost. For a POP3 account configured to remove messages from the server after download, the local file may be the only copy.

To find the OST file path, go to File > Account Settings > Account Settings, click the Data Files tab, and read the path column. If you need to rename or delete the file, close Outlook first.

How to reduce Outlook OST file size: step by step#

The five steps below run from the quickest and safest to the more involved. For most users, steps 1 through 3 are enough. Only move to step 5 if the earlier steps do not produce the result you need.

1. 1

   Delete unwanted mail and empty the Deleted Items folder

   Sort Sent Items and your largest folders by size — in classic Outlook, use View > Arrange By > Size. Remove messages with large attachments you no longer need. Right-click Deleted Items and choose Empty Folder, then do the same for Junk Email. Compacting before this step reclaims far less space than compacting after a thorough cleanup.

2. 2

   Run Compact Now in classic Outlook

   Go to File > Account Settings > Account Settings. On the Data Files tab, select your OST file and click Settings. For Exchange or Microsoft 365 accounts, also click the Advanced tab, then Outlook Data File Settings. Click Compact Now, then OK. The process may take several minutes; Outlook shows progress in the status bar at the bottom of the window. You can continue working while it runs.

3. 3

   Narrow the Mail to Keep Offline sync window

   Go to File > Account Settings > Account Settings. Select your Exchange or Microsoft 365 account and click Change. Under Offline Settings, drag the slider from its current position — likely All or 12 months — to 3 or 6 months. Click Next and Finish. Outlook removes older items from the local cache on the next sync. Those items remain on the server and are still searchable from Outlook; only the local copy is removed.

4. 4

   Archive old mail to a separate .pst file (optional)

   If you want older mail available offline but not in the active OST, go to File > Cleanup Tools > Archive. Set a date threshold and choose a destination .pst file. Outlook moves items older than that date out of the OST and into the archive file, which it continues to open alongside the main account. This reduces OST size without deleting anything from either location.

5. 5

   Delete and rebuild the OST from the server (last resort)

   Close Outlook. Navigate to the OST file path shown in Account Settings and rename or delete the file. Reopen Outlook — it begins downloading a fresh copy from the server. This is safe for Exchange and Microsoft 365 accounts; all mail is on the server. If you set the sync window to 3 months before triggering the rebuild, the initial download completes faster.

Classic Outlook vs. New Outlook: what works where#

Microsoft's New Outlook for Windows, released broadly in 2024, uses a different caching architecture from classic Outlook. It does not support Cached Exchange Mode in the traditional sense, so the Compact Now button and the Mail to Keep Offline slider are not available. If you are running New Outlook and cannot find these controls, that is why.

New Outlook has a simplified Offline page under Settings > General > Offline, which sets how many days of mail the client keeps locally: 7, 30, 90, or 180 days. The table below shows which methods apply to each version.

What to do when Compact Now does not work#

If Compact Now finishes without an error but the OST file size barely changes, three causes cover most cases. The most common is insufficient white space — Outlook did not find enough unused space to cross the 20 percent threshold. Delete more items, particularly from Sent Items and Deleted Items, and compact again. The second cause is a background compaction already in progress from an earlier session; look for a 'Compacting Data File' status message at the bottom of the Outlook window before starting another pass.

The third cause is file corruption. A corrupted OST can prevent compaction from completing cleanly or reporting accurate results. The fix is to run scanpst.exe — the Inbox Repair Tool that ships with Outlook — against the OST file while Outlook is closed.

Search for scanpst.exe in File Explorer to find the exact location for your Outlook installation, as the path varies across versions and whether you use a Click-to-Run or MSI install. After a successful repair pass, reopen Outlook and try Compact Now again.

If the OST is growing back quickly after each compaction, the sync window is more likely the issue than white space. A 12-month window on an active mailbox refills as fast as you compact it. Narrowing the window to 3 months limits the inflow permanently regardless of how often you compact.

A faster way: stop maintaining a local cache entirely#

The steps above work, but they require repeating. White space accumulates again as new mail arrives, and the sync window needs revisiting as your mailbox grows. Cached Exchange Mode was designed for slow or unreliable server connections. On a modern fast connection, mirroring months of mail into a 50 GB local file is often unnecessary overhead.

AI Emaily connects to Exchange and Microsoft 365 — alongside Gmail and IMAP providers — without maintaining a large local cache file on your drive. There is no OST to compact, no sync window to configure, and no file to rebuild when something goes wrong. We build AI Emaily, and it includes a 7-day free trial if you want to stop managing local storage altogether.

The limit is real and worth stating: AI Emaily is not a locally cached client and does not carry a full offline archive of years of historical mail. If offline access to a large historical archive is a firm requirement, classic Outlook with a narrowed sync window remains the right tool for that job.