User Outlook .OST file

For an Exchange or Microsoft 365 account, permanently control OST growth by reducing Mail to keep offline; if that option is unavailable, use the mailbox-cleanup and compaction path instead.

1. Verify which path applies

In classic Outlook, open File > Account Settings > Account Settings, select the account, and check its Type.

• Exchange or Microsoft 365: Select Change. If Offline Settings and the Mail to keep offline slider appear, continue below.
• IMAP or another account type, or no slider: Skip to Clean and compact the OST.

2. Reduce the cached-mail period

For Exchange or Microsoft 365:

1. Go to File > Account Settings > Account Settings.
2. Select the account and choose Change.
3. Under Offline Settings, change Mail to keep offline from All or 12 months to 6 months, 3 months, or 1 month.
4. Finish the wizard and restart Outlook.

Only mail within that period remains synchronized into the OST; older mail remains in the server mailbox and is still available when Outlook is online. Microsoft notes that reducing synchronized data lowers local storage use and can improve Outlook startup time.^1^

3. If shared mailboxes are attached

Apply this only if the profile contains shared mailboxes or shared folders. Their contents can also be downloaded into the same OST and increase it significantly.^2^

To stop caching shared mail folders:

1. Open File > Account Settings > Account Settings.
2. Select the Exchange account and choose Change.
3. Select More Settings > Advanced.
4. Clear Download Shared Folders.
5. Restart Outlook.

Shared folders will then operate online, so access may be slower and their mail contents will not be available offline.

If shared mailboxes continue downloading their complete history despite reducing the synchronization period, have the administrator check whether DisableSyncSliderForSharedMailbox is set to 1. That value makes the entire shared-mailbox content synchronize when shared-folder caching remains enabled.^2^

4. Clean and compact the OST

Reducing synchronized content does not necessarily shrink the physical OST immediately.

1. Open File > Tools > Mailbox Cleanup.
2. Use View Mailbox Size, Find items larger than, Empty Deleted Items, and Empty Conflicts as appropriate.
3. Then open File > Account Settings > Account Settings > Data Files.
4. Select the OST and choose Settings.
5. For Exchange or Outlook.com, select Advanced > Outlook Data File Settings.
6. Select Compact Now.

Outlook performs automatic background compaction only when the file contains more than 20% unused space, so manual compaction can reclaim space sooner.^3^

Why increasing the registry limit is not the durable fix

The Unicode settings are MaxLargeFileSize and WarnLargeFileSize; the latter stops additional data from being added once its threshold is reached. Microsoft recommends at least a 5% difference between warning and maximum values and warns that increasing Unicode limits beyond documented defaults can reduce performance.^4^

Large OST files can cause pauses well before the absolute limit—particularly above 10 GB, with more frequent pauses around 25 GB or larger.^5^ Therefore, reducing cached content is preferable to raising the ceiling again.

If “file currently in use” persists

Fully close Outlook before performing data-file or profile work. If Outlook still cannot open the OST:

1. Back up the existing data file where appropriate.
2. Run the Inbox Repair Tool (SCANPST.EXE) to check for data-file errors.
3. If repair does not resolve the problem, create a test profile through Control Panel > User Accounts > Mail > Show Profiles > Add.
4. Start Outlook with the test profile. If it works normally, remove the old profile only after verifying that all required mailbox data is available.

Microsoft documents data-file repair before new-profile creation in the classic Outlook troubleshooting sequence.^6^

References

1. Classic Outlook
2. Manage download settings for shared mail folders in Cached Exchange mode in Outlook
3. Reduce the size of your mailbox and Outlook Data Files (.pst and .ost) | Microsoft Support
4. Configure the size limit for both .pst and .ost files in Outlook
5. How to troubleshoot performance issues in Outlook | Microsoft Support
6. Classic Outlook not responding, stuck at "Processing," stopped working, or freezes | Microsoft Support