Outlook OST Corruption Symptom: Search Index Resync Without OST Rebuild

You open Outlook and notice the search bar returns no results or shows outdated emails. The status bar at the bottom of the Outlook window displays “Indexing complete” or “Indexing in progress” repeatedly. This symptom points to a corrupted Offline Outlook Data File (.ost) where the search index keeps resyncing but the OST itself is never rebuilt. This article explains why this specific corruption pattern occurs and how to force a proper OST rebuild to restore reliable search functionality.

Why a Corrupt OST Causes Search Index to Resync Without Rebuilding the OST

The Offline Outlook Data File (.ost) is a local copy of your Exchange Online or on-premises mailbox. Outlook uses this file to provide offline access and to feed the Windows Search index. When the OST becomes corrupt, the search index detects inconsistencies between the index and the OST data. Instead of triggering an OST rebuild, Outlook attempts to resync the index repeatedly. This creates a loop where the index starts, fails, and restarts without ever replacing the underlying corrupt OST file.

Common causes of OST corruption include abrupt Outlook shutdowns, disk write errors during sync, and large mailbox items that exceed the OST structure limits. The corruption affects only the local file, not the server mailbox. The search index resync symptom is often the first visible sign because the index depends on the OST’s internal data consistency.

How the Search Index Uses the OST

Windows Search reads the OST file through the Outlook MAPI provider. It builds a content index of email bodies, subject lines, attachments, and metadata. When the OST has a corrupt block, the indexer cannot read that portion, so it marks the index as incomplete. Outlook then triggers a resync to fetch the missing data, but because the OST itself is damaged, the resync fails. The cycle repeats until the OST is manually rebuilt.

Steps to Force a Full OST Rebuild and Restore Search

The only reliable fix for this symptom is to delete the corrupt OST file and let Outlook download a fresh copy from the server. Do not simply delete the search index, because the index will rebuild from the same corrupt OST and the problem returns immediately.

1. Close Outlook completely
   Make sure no Outlook processes are running in the background. Open Task Manager with Ctrl+Shift+Escape and end any Outlook.exe tasks if present.
2. Open the Mail applet in Control Panel
   Press Windows+R, type control panel, and click OK. Change the view to Large icons. Click Mail (Microsoft Outlook). If you use Outlook 365, the applet is named Mail (Microsoft 365).
3. Click Show Profiles and select your profile
   In the Mail setup dialog, click Show Profiles. Select the profile that is experiencing the issue. Click Properties.
4. Open Data Files and locate the OST
   In the profile properties, click Data Files. Select the entry with type Outlook Data File that points to an .ost file. Click Open File Location to see the full path. Note the path, then close the folder.
5. Remove the OST from the profile
   Back in the Data Files dialog, select the OST entry and click Remove. Confirm the warning that you are removing the data file. Click OK to close all dialogs.
6. Delete the OST file from disk
   Open File Explorer and navigate to the path you noted earlier. The default location is %localappdata%\Microsoft\Outlook. Delete the .ost file. If you see a .ost.bak file, delete that as well.
7. Restart Outlook
   Open Outlook. It will display “Setting up your account” and download a new OST from the server. This process can take several minutes depending on your mailbox size and network speed.
8. Rebuild the Windows Search Index for Outlook
   Go to File > Options > Search. Under Indexing, click Indexing Options. In the Indexing Options dialog, click Advanced. Under Troubleshooting, click Rebuild. Confirm the rebuild. This forces Windows Search to create a fresh index from the new OST.
9. Verify search works
   Wait for indexing to complete. Check the status bar in Outlook — it should show “Indexing complete”. Test search with a few known email subjects. Results should appear instantly.

If Outlook Still Has Search Issues After the OST Rebuild

Even after deleting the OST and rebuilding the index, some users still see search problems. These cases usually involve additional corruption in the Outlook profile or Windows Search configuration.

Outlook Search Still Shows “No Results” After OST Rebuild

If search returns no results but the index status says “Indexing complete”, the Windows Search database may have residual corruption. Open Indexing Options again, click Advanced, and under Index Settings, click Rebuild a second time. Then restart the Windows Search service by opening Command Prompt as administrator and running net stop wsearch followed by net start wsearch.

Outlook Keeps Prompting for Password After OST Rebuild

A corrupt OST can also damage the stored credentials in the profile. After rebuilding the OST, go to Control Panel > Credential Manager > Windows Credentials. Remove any entries under Generic Credentials that reference Outlook or Microsoft Office. Restart Outlook and sign in fresh.

Search Index Status Shows “Paused” and Never Completes

This occurs when Windows Search is low on resources or the index database is corrupt. Open Indexing Options and check if indexing is paused. Click Pause to resume. If it pauses again, run the Windows Search troubleshooter by going to Settings > Update & Security > Troubleshoot > Additional troubleshooters > Search and Indexing. Run the troubleshooter and apply the recommended fixes.

Item	Delete OST Only	Delete OST + Rebuild Index
Description	Removes corrupt OST and triggers a fresh download from the server	Removes corrupt OST and also clears the Windows Search content index for Outlook
Effect on search	Search may still show outdated results if the old index persists	Forces a completely new index, eliminating all residual corruption
Time required	OST download time plus initial sync	OST download time plus full index rebuild, which can take hours for large mailboxes
When to use	When search returns partial results or missing items	When search returns no results or the index resync loop continues after OST replacement

You can now identify the specific symptom of a corrupt OST causing a search index resync loop. The fix requires deleting the OST file from the profile and from disk, then rebuilding the Windows Search index. After completing these steps, test search with a variety of emails. If the problem persists, check the Windows Search service and credential manager for additional corruption. As an advanced step, you can also run Outlook.exe /cleanfreebusy from the Run dialog to clear cached free/busy data that may interfere with indexing in shared calendar scenarios.