How to Rebuild an OST File in Outlook
Rebuilding an OST file means letting Outlook recreate it from a healthy mailbox — it is not the same as converting the file to PST.
If the mailbox is still live and complete, rebuild the OST and let it resync. Preserve the old file first if it might hold data that never made it to the server, and recover that data separately only when the original mailbox isn't available.
Outlook stores a local, offline copy of your mailbox — emails, contacts, calendar entries, and attachments — in an OST (Offline Storage Table) file. This lets you keep working even without an internet connection; any changes you make sync back to the server once you're reconnected. Outlook creates this file automatically when your account is set up with Exchange, Office 365, or an IMAP server. The steps in this guide are for classic Outlook for Windows; New Outlook for Windows uses a different offline-storage model and isn't covered here.
When the OST file becomes damaged or unreadable, Outlook can start throwing sync errors, freezing, or failing to open the mailbox altogether. Rebuilding the file — letting Outlook recreate it from the server — is the standard fix. Server-synchronized data normally redownloads without issue; the real risk is losing anything that existed only in the local OST and hadn't synced yet, such as drafts or items received during an outage. Preserve the old file first if that's a possibility.
This guide covers what typically causes OST corruption, several ways to rebuild or repair the file in current versions of classic Outlook — from lightest-touch to most thorough — and how to recover data from a damaged or unreachable OST file before rebuilding, using EdbMails OST to PST Converter, so you don't lose anything in the process.
Common Causes of OST File Corruption
A few conditions account for most OST corruption and access issues:
- Large OST or constrained storage — a very large local cache can affect Outlook performance or reach configured limits, but size alone does not prove corruption. Adjusting the Cached Exchange Mode cache range or recreating the cache can be appropriate when the server mailbox is healthy.
- Sudden power failure or forced shutdown — if the system shuts down while Outlook is syncing the OST file, the file can be left in a corrupted state.
- Synchronization or connectivity problems — these can leave the local cache stale or incomplete, although a network interruption alone does not prove file corruption.
- Storage device issues — bad sectors or hardware faults on the drive where the OST file is stored can damage it.
- Outlook software issues — outdated Outlook builds, software bugs, or incompatible/faulty add-ins can also trigger OST corruption or sync failures.
Rebuild vs. Recovery: Which Do You Need?
| Situation | Recommended action |
|---|---|
| Live mailbox healthy and complete | Rebuild/recreate the OST from the server |
| Old OST may contain unsynced/local-only data | Preserve the old OST before rebuilding |
| Original mailbox unavailable | Recover the old OST instead of rebuilding |
| OST structurally damaged but mailbox healthy | Rebuild first; run ScanPST only if that alone doesn't resolve it |
Before You Rebuild: Preserve Unique Local Data
Before running any of the methods below, consider whether the OST might hold anything that never made it back to the server — drafts, items received while offline, or changes made during a network or server outage. If there's any chance of that, back up or copy the existing OST file first, and consider using EdbMails to scan and export its contents to PST before you rebuild, so that data isn't lost if the rebuild proceeds.
