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Sherlock OST Viewer Open OST Files Without Exchange

Employee left the company? Exchange decommissioned? Office 365 account disabled? Open the OST file anyway. SHA256 verified. CISSP certified. Since 2006.

Sherlock OST Viewer is a free Windows application that opens Outlook OST (Offline Storage Table) files without Exchange Server or Office 365. It bypasses the profile-lock restriction on cached mailbox files, letting you browse emails, contacts and calendar items from deactivated accounts. SHA256 hash verification included. Pro edition at $67 USD adds forensic reporting and chain of custody logging.

Also supports PST files. See our dedicated PST Viewer page.

Background

What Is an OST File?

An OST (Offline Storage Table) file is a local cached copy of an Exchange or Office 365 mailbox. Outlook creates it automatically so users can read and compose email while disconnected from the server. The OST syncs with Exchange when connectivity resumes.

Every Outlook installation connected to Exchange or Office 365 in cached mode creates an OST file on the local machine. This file contains a full copy of the mailbox: emails, contacts, calendar entries, tasks and notes. The OST file format uses the same underlying MS-PST binary structure as PST files.

The critical difference between OST and PST is portability. PST files are standalone archives you can copy between machines. OST files are locked to the Exchange profile that created them. Microsoft designed this restriction to prevent unauthorized access to cached mailbox data. In practice, it creates a serious problem when the Exchange profile no longer exists.

The Problem

Why OST Files Won't Open

Outlook refuses to open an OST file unless it can authenticate against the Exchange profile that created it. When that profile is gone, the data is trapped. Common scenarios where this happens:

Employee departure
The employee's Exchange or Office 365 account is deactivated. Their workstation still contains the OST file with months or years of cached email. Outlook will not open it.
Exchange server decommissioned
Organizations migrating from on-premises Exchange to Office 365 decommission the old server. OST files created by the old Exchange profiles become inaccessible.
Office 365 migration
Migrating between Office 365 tenants or switching from Exchange to another platform orphans existing OST files. The new profile does not match the old cache.
Litigation hold
Legal counsel issues a preservation order on employee email. The employee's account is already deactivated. The OST file on the workstation is the only remaining copy of cached correspondence.
Account compromise recovery
A compromised account is disabled for security. The local OST file preserves the mailbox state before and during the compromise for forensic analysis.

Solution

How Sherlock Opens OST Files

OST and PST files share the same underlying binary format: MS-PST. Sherlock reads this format directly using its own parser. It does not rely on Outlook, Exchange or any Microsoft component. When you open an OST file, Sherlock auto-detects the format, parses the node database and B-tree structures, and presents the full folder hierarchy with all messages, contacts and calendar items.

The same executable handles both OST and PST files. There is no separate download. The viewer identifies the file type automatically and loads accordingly.

Note: Very recent Office 365 cached OSTs with Exchange-specific encryption may not parse fully. Standard cached-mode OST files from Exchange 2010 through Office 365 open without issue. If you encounter an encrypted OST, contact us for assistance.

Compare

Free vs Pro

FeatureFreePro ($67)
Open OST/PST filesYesYes
View emails, contacts, calendarYesYes
Search within OSTYesYes
SHA256 hash verificationYesYes
Export to EML/MSGNoYes
Batch exportNoYes
Forensic report generationNoYes
Chain of custody loggingNoYes
Priority supportNoYes
Court-ready PDF reportsNoYes
Per-message SHA-256 hashingNoYes
Sender IP attributionNoYes

Pricing

One-Time Payment. Yours Forever.

Pro License

$67 USD
Single machine license. No subscription. One-time payment. Yours forever.
  • All free features included
  • Export to EML/MSG formats
  • Batch export entire OST archives
  • Forensic report generation
  • Chain of custody logging
  • Priority email support
  • 30-day money-back guarantee

5+ machines? Contact us for volume pricing.

Checkout - OST Viewer Pro

$67.00 USD. One-time payment. License key delivered to your email.

Secure checkout via Stripe 30-day money back No subscription

Use Cases

Who Uses Sherlock OST Viewer

IT Administrators

An employee leaves and their Office 365 license is reclaimed. Six months later, someone needs an email from that person's mailbox. The OST file is still on the old workstation. Sherlock opens it without reactivating the account or purchasing another license.

Litigation Support

Legal hold requires preserving all email from a custodian whose Exchange account was deactivated before the hold was issued. The OST file on their machine is the only copy. Sherlock opens it in read-only mode with SHA256 verification for evidentiary integrity. Pairs with our expert witness services.

Exchange Migration Teams

Decommissioning on-premises Exchange for Office 365 leaves orphaned OST files across hundreds of workstations. Sherlock lets migration teams verify that cached data matches the migrated mailboxes before formatting old machines.

Forensic Examiners

Incident response often requires examining cached email from compromised or disabled accounts. The OST file preserves the mailbox state at the time of compromise. Sherlock provides read-only access with hash verification for forensic investigations.

HR Investigations

Internal investigations into harassment, policy violations or data theft frequently involve reviewing a former employee's email. When the Exchange account is gone, the OST file is the only remaining source. Sherlock opens it without involving IT infrastructure changes.

Guide

How to Open an OST File

  1. Download Sherlock OST ViewerDownload the free installer from this page. Under 5 MB. SHA256 hash: 074cdb29d0202bc64faaac5ba07d00b2a7e7bb5c9cd269b28c4b3f71bc530d5c
  2. Install and LaunchRun the installer on Windows 10 or 11. No admin privileges required. Launch from the Start menu.
  3. Locate the OST FileOST files are typically stored in C:\Users\[username]\AppData\Local\Microsoft\Outlook\. Look for files with the .ost extension.
  4. Open and BrowseClick Open File and select the OST file. Sherlock auto-detects the MS-PST format and loads the folder structure. Browse cached Inbox, Sent Items, Contacts and Calendar.
  5. Search, Verify and ExportUse full-text search to find specific emails. View SHA256 hashes for integrity verification. Pro users can export to EML/MSG and generate forensic reports.

Questions

OST Viewer FAQ

What is an OST file?
An OST (Offline Storage Table) file is a local cached copy of your Exchange or Office 365 mailbox. Outlook creates it automatically so you can work offline. It stores emails, contacts, calendar items and tasks. Unlike PST files, OST files are tied to the Exchange profile that created them and cannot be opened on another machine through Outlook.
Why can't I open an OST file in Outlook?
Outlook requires the original Exchange or Office 365 profile to open an OST file. If the account has been deactivated, the server decommissioned or the profile removed, Outlook will refuse to open the file. Sherlock OST Viewer bypasses this restriction by reading the MS-PST binary format directly.
Do I need Exchange or Office 365 to use Sherlock OST Viewer?
No. Sherlock OST Viewer opens OST files without Exchange, Office 365 or Microsoft Outlook. It uses its own parser to read the file format directly. No active mailbox, no Microsoft license, no server connection required.
Can Sherlock open Office 365 OST files?
Sherlock opens most Office 365 cached OST files without issue. Very recent Office 365 cached OSTs with Exchange-specific encryption may not parse fully. If you encounter a file that does not load, contact us for assistance.
Is the OST Viewer the same download as PST Viewer?
Yes. The same executable handles both OST and PST files. OST and PST share the same MS-PST binary format. The viewer auto-detects which type you opened and loads it accordingly. One download, both formats.
Can I recover emails from a former employee's OST file?
Yes. Locate the OST file on the former employee's workstation (typically in AppData\Local\Microsoft\Outlook). Open it with Sherlock OST Viewer. You can browse and search all cached emails without reactivating the Exchange or Office 365 account.
Is the $67 price a subscription?
No. The $67 USD Pro license is a one-time payment. No subscriptions, no recurring charges. You own the license permanently with free updates included.
Can I use this for litigation hold?
Yes. Sherlock OST Viewer opens files in read-only mode, preserving evidentiary integrity. The Pro edition generates forensic PDF reports with per-message SHA-256 hashing and chain of custody documentation. Built by CISSP, ISSAP, ISSMP certified examiners with courtroom experience.

Get Started

Download Sherlock OST Viewer Today

Free for viewing, searching and hash verification. Pro at $67 USD for export, forensic reports and chain of custody logging. Built by the same team that delivers expert witness testimony and forensic investigations in Canadian courts.

Since 2006CISSP, ISSAP, ISSMP certified604.229.1994

Sherlock OST Viewer is provided for lawful use. Terms of Service

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