What Is an OST File?
An OST file — Offline Storage Table — is a local cache file created by Microsoft Outlook when you connect to an Exchange Server, Microsoft 365, or an IMAP email account. It contains a synchronized copy of your mailbox, allowing you to read, compose, and organize emails even when you are not connected to the network. Changes you make offline sync back to the server once connectivity is restored.
Every Outlook user connected to an Exchange or Microsoft 365 account has an OST file on their computer, whether they know it or not. The file is created automatically and maintained by Outlook, sitting quietly in the background while you work with your email.
Understanding OST files becomes important when something goes wrong — a corrupted profile, a decommissioned server, a departed employee — and you suddenly need to access data that is trapped in an OST file that Outlook refuses to open.
How OST Files Work
The Synchronization Mechanism
When Outlook connects to an Exchange or Microsoft 365 server, it establishes a synchronization relationship:
- Initial sync — Outlook downloads the entire mailbox (or a configured portion) to the local OST file
- Ongoing sync — As new messages arrive on the server, they are copied to the OST file. Changes you make locally (read/unread, moves, deletes) sync back to the server.
- Offline mode — When the connection drops, Outlook switches to the OST file seamlessly. You can continue working with your email.
- Reconnection — When connectivity resumes, Outlook reconciles differences between the OST file and the server, applying changes in both directions.
Cached Exchange Mode
Cached Exchange Mode is the Outlook feature that manages OST files. You can configure how much data is cached locally:
- Last 1 month — Only recent email is stored locally
- Last 3 months, 6 months, 1 year, 2 years — Increasing amounts of history
- All — The entire mailbox is cached
The more data you cache, the larger the OST file grows, but the more data is available offline.
Where OST Files Are Stored
By default, OST files are located at:
- Windows 10/11:
C:\Users\<username>\AppData\Local\Microsoft\Outlook\ - Older Windows:
C:\Users\<username>\AppData\Local\Microsoft\Outlook\(may vary)
The file is typically named after the email account, such as john.doe@company.com - Exchange.ost or similar.
OST vs. PST: Understanding the Differences
While OST and PST files share the same underlying binary format (the MS-PST specification), they serve fundamentally different purposes:
| Feature | OST | PST |
|---|---|---|
| Purpose | Offline cache of server mailbox | Standalone local archive |
| Created by | Outlook automatically | User manually (export/archive) |
| Tied to account | Yes — specific profile and server | No — portable between computers |
| Can be moved between PCs | No | Yes |
| Sync with server | Yes — bidirectional | No — standalone file |
| Accessible without original account | No (without conversion) | Yes |
| Used with | Exchange, Microsoft 365, IMAP | POP3, manual archives, exports |
| What happens if server is gone | File becomes orphaned | File continues to work |
The critical distinction: PST files are self-contained and portable. OST files are dependent on their parent account. Remove the account, and the OST becomes an orphan — the data is inside, but Outlook will not open it.
When OST Files Become a Problem
Orphaned OST Files
The most common OST problem occurs when the parent account becomes unavailable:
- Employee departure — The Exchange account is disabled or deleted, but the OST file remains on the workstation
- Server decommission — An on-premises Exchange server is shut down, and the transition to Microsoft 365 did not capture all cached data
- Profile corruption — The Outlook profile is damaged, and recreating it generates a new OST file, leaving the old one inaccessible
- Account removal — The email account is removed from Outlook, either intentionally or by accident
In all these cases, the email data is physically present in the OST file but cannot be accessed through Outlook.
Corrupted OST Files
OST files can become corrupted due to:
- Unexpected shutdowns during sync
- Disk errors
- Outlook crashes
- Antivirus interference
- Network interruptions during synchronization
Symptoms include Outlook errors on startup, missing folders, blank message bodies, and crashes when accessing specific items.
Oversized OST Files
OST files grow as your mailbox grows. With Microsoft 365 mailboxes that can reach 50 GB or more, OST files can become unwieldy:
- Outlook performance degrades with very large OST files
- Disk space on laptops may be insufficient
- Backup tools may struggle with multi-gigabyte files
- Compacting (which releases unused space) can be extremely slow
How to Access Orphaned OST Files
Why You Cannot Just Open an OST File
Outlook checks the OST file’s internal account identifier against the configured mail profile. If they do not match, Outlook refuses to open the file. You cannot trick Outlook into opening someone else’s OST file by renaming it or copying it to a different location.
Solution: Convert OST to PST
The standard approach to recovering data from an orphaned OST file is to convert it to PST format. A PST file is not tied to any account and can be opened in any copy of Outlook.
Convert OST to PST using MailtoPst:
- Upload the OST file
- MailtoPst processes the file on secure EU servers
- Download the resulting PST file
- Open the PST in Outlook or any compatible tool
Alternative: Convert to Universal Formats
If you do not have Outlook or need the data in a non-Microsoft format:
- Convert OST to EML — Individual message files readable by any email client
- Convert OST to MBOX — For Thunderbird and other MBOX-compatible clients
- Convert OST to MSG — Individual messages in Microsoft format
Repairing OST Files
When to Repair vs. Recreate
If the Exchange/Microsoft 365 account is still active, the simplest fix for a corrupted OST file is to delete it and let Outlook create a new one:
- Close Outlook
- Navigate to the OST file location
- Rename or delete the OST file
- Reopen Outlook — it will create a fresh OST and resync from the server
This works because the server has the authoritative copy of your mailbox. The OST is just a cache.
When the Server Is Unavailable
If the server is gone and the OST file is your only copy of the data, you cannot recreate it. In this case:
- Do not delete the OST file
- Make a backup copy immediately
- Convert the OST to PST to extract as much data as possible
- If the file is corrupted, try conversion tools that include repair capabilities
Microsoft’s OST Integrity Check
Outlook includes a built-in integrity check for OST files that runs automatically. If it detects problems, it may:
- Repair minor inconsistencies silently
- Prompt you to run the Inbox Repair Tool
- Delete and recreate the OST file (if the server is available)
OST Files in Enterprise Environments
Managing OST Storage
In organizations with thousands of users, OST files consume significant disk space:
- A typical knowledge worker’s OST file is 2-10 GB
- Heavy email users or executives may have 20-50 GB OST files
- Multiply by hundreds or thousands of employees, and disk usage becomes substantial
Strategies for managing OST storage:
- Limit cached period — Configure Cached Exchange Mode to sync only recent months
- Use OneDrive known folder move — Ensures OST files are excluded from cloud sync
- Roaming profile exclusions — Exclude OST files from roaming profiles (they must be recreated anyway)
- Disk space monitoring — Alert when OST files exceed a threshold
OST Files During Employee Offboarding
When an employee leaves the organization:
- Before disabling the account — Export the mailbox to PST on the server side using Exchange admin tools
- After disabling the account — The employee’s OST file becomes orphaned. If the server-side export was not done, convert the OST to PST from the workstation
- Retention — Store the PST archive according to your organization’s retention policy
- Cleanup — Delete the orphaned OST file from the workstation
Legal Hold Considerations
OST files can contain data not present on the server — specifically, items the user modified or composed offline but never synced. In litigation hold scenarios:
- Preserve OST files alongside server-side mailbox data
- Compare OST content with server mailbox to identify discrepancies
- Convert OST to EML for individual message review and production
IMAP OST Files
How IMAP OST Files Differ
Outlook also creates OST files for IMAP accounts, but these behave differently from Exchange OST files:
- IMAP OST files cache only the messages and folders visible via IMAP
- They do not contain contacts, calendars, or tasks (these are in a separate data file)
- Sync is simpler — IMAP is a read-focused protocol compared to Exchange’s full synchronization
IMAP to Exchange Migration
When migrating from IMAP to Exchange/Microsoft 365, the old IMAP OST file becomes unnecessary. However, if the IMAP server is being decommissioned and the OST file contains data not yet migrated:
- Convert the IMAP OST to PST before the server goes offline
- Import the PST into the new Exchange/Microsoft 365 mailbox
- Delete the old IMAP OST file
OST File Security
Encryption
OST files use the same encryption levels as PST files:
- No encryption — Data stored in clear text
- Compressible encryption — Basic byte-substitution (default)
- High encryption — Slightly stronger substitution
As with PST files, these encryption methods do not provide meaningful security by modern standards.
Physical Security
Since OST files contain a complete copy of the user’s mailbox, physical access to the computer means access to all their email. This is a concern for:
- Stolen or lost laptops
- Decommissioned hardware not properly wiped
- Shared workstations
Mitigations include BitLocker (Windows) full-disk encryption, and policies to securely wipe devices before disposal.
Remote Wipe
Microsoft 365 and Exchange support remote wipe capabilities. If a device is lost or stolen, administrators can remotely trigger the removal of the OST file (along with other corporate data) from the device.
OST Files and Migration to Microsoft 365
On-Premises Exchange to Microsoft 365
When migrating from on-premises Exchange to Microsoft 365:
- OST files are not used for migration — the data migrates server-to-server
- After migration, Outlook creates new OST files connected to the Microsoft 365 mailbox
- Old on-premises OST files become orphaned and should be deleted
- In rare cases where server-side migration missed data, the old OST file may be the only source
Hybrid Environments
In hybrid Exchange environments (some mailboxes on-premises, some in Microsoft 365):
- OST files connect to whichever server hosts the mailbox
- Moving a mailbox from on-premises to cloud triggers OST recreation
- The transition should be transparent to the user
Frequently Asked Questions
Can I open an OST file without the original Exchange account?
Not directly in Outlook. OST files are tied to their original Exchange/Microsoft 365 account and Outlook profile. To access an orphaned OST file, you need to convert it to PST or another portable format. MailtoPst can handle this conversion online without any software installation.
Can I convert an OST file to PST?
Yes. OST to PST conversion is the standard method for extracting data from orphaned OST files. MailtoPst processes the conversion on secure EU servers with GDPR compliance and automatic file deletion after 24 hours.
What is the maximum size of an OST file?
The default maximum OST file size is 50 GB, matching the PST limit. This can be adjusted through Windows registry settings. However, performance degrades significantly with very large OST files, and Microsoft recommends limiting cached data to what is actually needed offline.
Should I back up OST files?
Generally, no. OST files are a cache of server data and can be recreated by syncing with the server. Backing up OST files wastes storage and backup time. The exception is if you suspect the OST may contain unsynchronized changes — for example, if the server crashed before changes were uploaded.
Can I move an OST file to another computer?
Moving an OST file to another computer does not work. The file is tied to the specific Outlook profile and machine that created it. If you need to move email data between computers, export to PST from the server side or use OST to PST conversion.
How do I reduce the size of my OST file?
- Reduce the Cached Exchange Mode period (e.g., from All to 12 months)
- Delete unnecessary emails and empty the Deleted Items folder
- Compact the OST file: File > Account Settings > Data Files > select the file > Settings > Compact Now
- Empty the Recoverable Items folder (if your organization allows it)
Is it safe to delete an OST file?
Yes, if the Exchange/Microsoft 365 account is still active. Deleting the OST file simply removes the local cache — Outlook will recreate it by syncing with the server. Do not delete the OST file if the account is no longer available and the OST is your only copy of the email data.