SysInfo OST Viewer Tool
Open and View OST Files on Windows/Mac OS With/Without Microsoft Outlook
*Trusted for Viewing Complex Data Files and Inspecting Email Data Precisely.
Open and View OST Files on Windows/Mac OS With/Without Microsoft Outlook
*Trusted for Viewing Complex Data Files and Inspecting Email Data Precisely.
OST ≠ PST. Every user is well aware that OST files are bound to the specific profile they were created in. Once the original Outlook, Exchange, or Microsoft 365 account is decommissioned, deprovisioned, credentials expire, or the application is uninstalled, OST files become inaccessible via Microsoft. The problems become severe when no built-in “Import” option exists, ScanPST.exe doesn’t work for OST, and recreating the profile isn’t a choice without an active account.
Here comes the hero software: the SysInfo OST Viewer Tool, which directly reads the OST file contents, bypassing the need for MS Exchange completely. It provides full visibility into all the contents within the OST file, including emails, contacts, calendar, notes, tasks, etc., and then exports it to the desired format.
“There is no officially supported method to access orphan OST files once the original MS Exchange or Microsoft 365 account/profile has been deleted or decommissioned. Moreover, the native Microsoft Outlook cannot open and view OST either.”
This is exactly what the OST File Viewer was built to deal with securely and accurately.
Deprovisioning workflows or performing forensic investigations, explore why OST Reader is the ideal user choice.
Scan and render every OST data type associated with the mailbox without hiding, skipping, or truncating files.
| Data Type | Description |
|---|---|
| Emails | Subject, sender, recipients, body content, inline and embedded images and objects, all with preserved metadata (To, From, CC) |
| Attachments | Documents, spreadsheets, images, ZIP archives, videos, and all other files attached to the OST emails can be extracted separately or kept inline and previewed before saving. |
| Contacts and Calendar | Full name, nickname, email address, phone/mobile number, company details, department, anniversary, birthday, notes, photos, distribution list membership, appointment, meeting, recurring events, reminder, organizer, location, date/time, attendee, and response details |
| Notes and Tasks | Memos, quick/sticky notes, timestamps, subject, class, folder, task title, description, start/end date, time, priority, status, categories, assigned to, and completion percentage |
| Folder Structure | Original mailbox hierarchy structure, including Inbox, Sent Items, Drafts, Deleted Items, Junk, Archives, and all custom user-created folders and subfolders |
| Save Recovered OST Files | Message-ID, chain, authentication results (SPF/DKIM/DMARC), Return-Path, Subject details, and routing metadata (Full RFC 5322) |
| MIME Structure | Multipart data structures, type of encoding, identifier boundaries, inline relationships, and content-type declarations |
Identify the common scenarios where using the OST Reader Tool is beneficial for all techies and non-techies.
Individual User, IT Professionals, & SysAdmins
Outlook crashed between sessions and cannot be re-launched. M365 subscription expired? Employee offboarded, and account is deprovisioned? OST becomes orphaned. In all these cases, SysInfo OST Viewer Software directly reads the file and exports mailbox emails to PST/EML with or without account access. No reinstallation, account reactivation, or subscription renewal required.
Digital Forensic Investigators & Document Managers
Migrating platforms and need to validate the OST file archives? Analyze email evidence without altering the original data during viewing, reading, or exporting to PST, MBOX, CSV, or other formats. Preview raw headers, MIME data, timestamps, and other body content, attachments, and metadata with read-only access without overwrites or parallel Exchange infra for forensic chain of custody
Legal, Compliance, & Audit Retention Users & Teams
Extract OST emails in PDF format to meet the GDPR, ISO 27001, and other legal compliance, audit, and industry standards. Filter data by date, metadata, or attachments and skip duplicates, while accessing OSTs without an Outlook or Exchange licence or from M365 cache archive local copies. Generate log reports for compliance documentation while keeping the headers and data integrity intact.
Locate specific messages or attachments quickly and easily from your OST mailboxes using the advanced viewing options available in the SysInfo OST Viewer Tool. No longer will you need to manually sift through the large number of files to find what you are looking for. Get complete access to your data, even if you do not have a native version of MS Outlook or Exchange. The dedicated OST File Viewer Tool is equipped with a rigorous set of search algorithms offering a multitude of filtering criteria to help retrieve the desired content from complex nested folder structures.
To find particular email messages, use the Email List Search by a particular keyword or filter options in the Smart Search, as listed below:
You can even choose the Open in Browser option to view files on the web if you cannot view them within the software. This option is available within the Body tab.
Key Note: The OST Viewer Software only displays 20,000 email messages at once in the preview pane.
Go through the advanced features of OST Viewer with no configurations or learning curve.
Ternary OST File Upload Criteria
The Load Outlook OST option automatically fetches the OSTs present on your system linked to the Outlook/Exchange profiles. Drag and Drop or Choose Manually allows you to browse the local drive, external disk, or network path to opt for OST, including all healthy, minimally corrupted, oversized, blank, orphaned, and password-protected email files from your mailboxes with or without an active profile.
Open OSTs With Zero Limits
Review the OST emails with single or batch uploads simultaneously to check all the email contents, attachments, and metadata, all while mirroring the original folder organization. SysInfo OST File Viewer allows easy movement across several folders and enables you to view upto 20,000 emails in a single folder batch at the same time. Validate the data integrity and fidelity preservation for enterprise compliance and eDiscovery.
6-Tab Forensic Email Verification
Every chosen folder from the tree structure view presents a list of emails which can be previewed across multiple tabs, including: Body, Attachments, Notes, Header, MIME, and Raw HTML. Achieve complete mailbox forensics with complete integrity within one click in the software or on the web via the Open in Browser option.
High-Precision Multi-Criteria Search
Search across entire OST archives with the dependable OST Reader. Use the Email List option to locate emails by a specific keyword, or try the Smart Search using From, To, Subject, Has Attachments, and Date Range altogether or individually to locate specific emails. Combine these filters for precise results in massive datasets.
Comprehensive 6 Export Formats
SysInfo OST Viewer Tool expands its algorithms and offers data export to PST, PDF, EML, MBOX, CSV, and MSG formats, other than email previews. It is ideal to maintain data records for access across platforms and for use in court submissions, compliance records, or analysis with a full folder hierarchy and metadata preservation.
Standalone Multilingual Engine
Access complete visibility of your OST emails and attachments in a preferred language, say, English, French, German, Italian, Spanish, and Japanese. Try the independent tool that facilitates OST preview with or without the active Exchange, M365, or MS Outlook profile with local offline access without any server uploads.
Flexible Bulky Attachment Control
4 different options are available with the intelligent file viewer software to handle email attachments with ease. Choose to keep attachments, save as pages, extract attachments separately, or exclude attachments during the export to a desired format. Reduce the output size for optimization while maintaining the originality of data elements.
Versatile Email Duplicate Removal
SysInfo OST File Reader enables smart deduplication of similar OST files before exporting to another format for data backup and safety. Identify and skip redundant email copies based on different criteria to achieve a clutter-free mailbox from a pool of large Exchange caches. Keep the original files clean and organized for future use without manual reviews.
Chronological Email Extraction
Date Filter of the amazing SysInfo Freeware Reader Tool provides 3 options: Save All, Include Selected Dates, and Exclude Selected Dates for filtering. Pick a Start and End Date to perform a selective export of OST files after preview for day-level precision.
Resumable Data Functionality
Have an interrupted export? Power outages, system restarts, or manual stops, use the Skip/Resume Export filter on re-run to achieve no duplication and zero data loss. Automatically identify complete files, skip, and resume for the rest.
Custom Keyword Filtering
Use the Best OST Viewer Text Filter to divide emails by To, From, and Subject for Contain, Not Contain, and All texts mentioned in the emails. Available with OST to PST and MBOX exports, the filter enables you to save time and space for significant purposes.
Dynamic Size-Based Filtration
Converting OST to PST? Want to split your oversized files? Use the Split PST File option to divide large PST files by MB/GB. Automatically split large PSTs after 45 GB with no size limits mentioned, and give a name to store them for easy identification.
High-Fidelity Conversion to PDF
Securing your Exchange OST emails as PDF for documentation and review? Choose to save emails as a Single PDF or Merge PDF to combine many into one, making it ideal for legal, compliance, regulatory, and eDiscovery archive records.
Custom Folder Name & Destination
Browse through the complex aspects of the OST File with the OST File Viewer Online by SysInfo, and apply a Custom Folder Name to save emails as desired. Otherwise, they are represented by Day, Date, and Time Folder at your chosen export destination.
Verifiable Compliance Logs
Prepare detailed processing reports automatically and allow you to access them with the View Logs option, necessary for audits. Document all the preview and export operations and check them scalably with the user-friendly interface of the easy-to-use software. Built for all technical and non-technical users, handling analysis and compliance tasks professionally through a single interface.
Cross-Platform Support & Evaluation
SysInfo Best OST Reader Tool supports all OSTs from Outlook and MS Exchange versions. Fully accessible on any version of Windows and Mac OS, whether it be 11, 10, 8.1, 8, 7, XP, Vista, or Apple Silicon Mac (M1, M2, M3, and so on), without relying on the local application or active profile in which the file was created. Run pilot tests with the Demo Version to evaluate the functionalities and export 100 emails per folder for free.
Go through the specifications of this SysInfo PST Viewer Tool so that you can install it quickly.
| Version: | 26.06 |
| Size: | 204 MB |
| License: | Multiple User |
| Release Date: | 15 June 2026 |
| Edition: | Single, Admin, Technician & Enterprise |
| Processor: | Intel® Core™2 Duo E4600 Processor 2.40GHz |
| RAM: | 4 GB (8 GB Recommended) |
| Disk Space: | Minimum Disk Space - 1 GB |
| Supported Windows: | 11, 10, 8.1, 8, 7, 2008, 2012 (32 & 64 Bit), and other Windows versions. |
| Supported Outlook: | Outlook 2024 (LTSC), 2021, 2019, 2016, 2013, 2010, 2007, 2003, 2002, 2000, and Outlook Express 4.0–6.0 (ANSI and UNICODE) |
The free trial release of SysInfo OST Viewer offers preview, backup, and export upto 100 OST emails per folder. To have limitless conversions, purchase the premium version of the software and take advantage of handling OST and PST all in one interface. Worried about compatibility, folder layout, or originality of your OST files? Demo provides enough information to decide after use.
Follow the given stepwise procedure to verify and secure OSTs without Outlook/Exchange.
Note: If you wish to save OST in several other image/document formats or need to directly import them to an email client, get the all-in-one SysInfo OST Converter!
SysInfo's Award-Winning Software Recognized by Experts- Highly Rated in the Best Category
TechRadar recognizes SysInfo OST File Reader software as the most trusted solution for quick review and analysis. It has been awarded the Best OST Tool Award 2026 across countries for fast analysis, deep email fidelity checks, access to the software offline and on the browser, with export functionalities. Self-explanatory software for all technical and non-technical users, facilitating a swift and scalable PDF, CSV, and other downloads with zero data loss and risks. . View more
Cnet rates the OST Reader Tool by SysInfo as 4.5/5 rating for 100% malware-free data access, intelligent preview, and smart filters handling during export to 6 different formats. With complete centralized data control, it allows reliable email body, attachments, metadata, read/unread status, timestamps, MIME, and raw header checks for verification across individuals, IT teams, and professionals. View more
33 Orphaned OSTs| Zero Active Exchange Profile| 255K+ Emails| Limited Timeframe
Case: The mid-sized financial services firm was planning a post-merger consolidation during which they found 33 orphaned OSTs and a few PSTs from a former executive Exchange account. The removal of the account from the Active Directory had already commenced 6 months before.
Challenge: Their current migration to Microsoft 365 required an active Exchange profile to process OST/PST and convert them to PST via Outlook. However, in the absence of the original profile, all files became inaccessible, and Outlook couldn’t open them.
Solution:
SysInfo OST Viewer instantly reads all the 33 files comprising 255K+ emails in one go without the need for the active Exchange directory. The Smart Search feature allowed the team to filter emails specifically by To, From, Subject, and Date across all mailboxes simultaneously. The entire dataset of the business-critical emails was saved to PST within a minimal timeframe, and the migration to M365 took place with zero risks or downtime. All the email integrity and folder structure were kept intact throughout the view and export process precisely.
Thomas Walters, IT Head of the Organization at Frankfurt, stated, “We had 33 orphaned OST and a few PST files from departed executives' accounts that were already removed from Active Directory. Every standard tool to open the OST file failed. SysInfo OST Viewer read every single one, and we had the data in Outlook PST format within no time with 100% accuracy and no metadata loss.”
Get to know how the SysInfo OST Reader trial and licenced version differ for users
| Features | Trial Version | Premium Version |
|---|---|---|
| Outlook and Exchange Dependent | ||
| Batch view Outlook offline email with attachments | ||
| Access all OST files without a size limit | ||
| Multiple Language Support | ||
| Export to PDF, MBOX, CSV, PST, EML, MSG | Only the first 100 mails/folder | All |
| Automatic Outlook Profile/File Detection | ||
| Drag and Drop OST Loads | ||
| View 20,000 emails at once | ||
| Smart Search Filtering | ||
| Split Large PST after 45 GB automatically | ||
| Handle Duplicates and Attachments | ||
| Custom Folder Name | ||
| Date Range & Text Filter | ||
| Skip Existing/Resume Export | ||
| Generate Activity Logs | ||
| 100% Safe & Secure | ||
 |
Download | Buy Now |
OST files differ from PST files by a cryptographic binding to the Exchange or M365 account from which the file was created. Because of this, Outlook cannot access the OST file without access to a matching live account.
The ScanPST.exe program cannot read OST files either. The only way to recreate your Outlook profile and access the OST file is if your server account is still active. An account that has been deleted cannot be recreated and will make the OST permanently inaccessible via any native method. The SysInfo OST Viewer does not rely on credentials from the server to read the underlying information. Thus, allowing users to access the contents of the OST file without having to authenticate with the server.
SysInfo is a globally verified software solution for email analysis, preview, and export on the local OS. It ensures that your data never leaves your machine and thus operates 100% offline without any telemetry cloud processing. The mailbox data within your OST files stays on your system, and any format changes within the OST with Outlook, Exchange, or M365 updates or account decommission don’t affect its originality. All the data within the OST folders and subfolders is kept intact throughout the preview and download process, with or without the native application. The fast speed, intelligent scanning, read-only capabilities, and archive logs ensure no lag or loss happens with the OST archives.
Answers to the most common user queries related to the OST Viewer Tool.
OST at the structural level uses the same NDB (Node Database B-tree) structure as PST, along with a cryptographic binding at the file header. When Outlook tries mounting the OST, it verifies it against the active Exchange profile, which, if unmatched, Outlook refuses to open it like a PST. SysInfo OST File Viewer bypasses this authentication layer, enabling OST data access without any significant active Outlook or Exchange profile/live session validation.
ANSI OST is a 32-bit file used with Outlook 97-2002 with a limit of 2 GB. UNICODE OSTs, which are 64-bit files accessible with Outlook 2003 to the latest, including M365, have a limit of 50 GB. ANSI files don’t store non-Latin scripts or Unicode multilingual email content. This is where OST Reader comes in, ensuring you can view both ANSI and UNICODE OSTs of 50 GB, 65 GB, or even more with no restrictions and structurally intact data in the file.
Ans: The OST file is by default stored at the C:\Users\%username%\AppData\Local\Microsoft\Outlook\. If you cannot locate it here, the file is probably redirected by the admin via ForceOSTPath, or the account was used in Online Mode with no OST Creation. The prominent SysInfo OST Viewer Software allows you to Choose OST Files Manually by browsing to any path (local drive, external disk, or network location) or even locate automatically with the Load Outlook OST. So, no barriers for the default path being non-standard when loading or opening the .ost file for preview.
Ans: You may copy the OST file at the OS level, but Outlook doesn’t mount it on another machine. Thus, often makes it inaccessible in the absence of the original path or account. With this file viewer tool, you can access very large OSTs and view their entire email body content with bulky attachments, verifying the integrity and folder structure.
Ans: Mostly, items in the Deleted folder are synced on the server and remain in OST until the folder is entirely deleted. The general time period to retain it manually is 14 days. You can use the OST Reader in case you want to restore them with full integrity preserved. The software is equipped with PST, EML, MSG, MBOX, CSV, and PDF formats in which you can export restored OST emails.
Ans: Once the original profile is deleted and recreated, the OST from the previous profile becomes orphaned and inaccessible. But the SysInfo OST Viewer enables you to access all orphaned or oversized OSTs, independent of Outlook's current profile configuration, providing full data access.
Ans: Whether you are a techie expert or belong to a non-technical background, the software is self-explanatory with an interactive interface needing no skills to use it for OST file viewing and export.
Ans:The software processes completely offline on the local OS with no server uploads or connection to the third-party application. It fetches files from your system, enabling their complete preview in read-only mode with no overwrites, allowing you to inspect the data integrity and structure preservation. Later, exporting to another format of choice with advanced filters on your OS, only with complete control.
Ans: No, because the data within the local OST file remains unaffected by the status of your M365 subscription. The file stays at the default path regardless of whatever happens on the server side. Though Microsoft has a specific retention period for expired accounts (30 days), in disable state (30-90) before permanent removal. It doesn’t affect OST at all, and all the data remains intact, easily accessible with SysInfo OST File Viewer without Exchange or M365 authentication.
Ans: There are no specific size limitations for the OST file you upload. It supports all OSTs in MB, GBs, or even TBs. However, you can only check upto 20,000 emails from a folder in one go in the preview panel. Also, the demo version supports exporting only the first 100 emails per folder. For a complete experience, upgrade to a Single, Admin, Technician, or Enterprise licence based on your organization’s size.
Ans: If your process gets interrupted in between for any reason, you can use the Skip/Resume export, and it will perform an incremental sync. The OST Reader will automatically scan the exported files and skip them. Only incomplete or new OSTs will be exported in the same location without duplicates.
Ans: This software is suited for all Windows, Mac, and Outlook environments and allows you to have a simplified view of your 80 GB OST emails with precision. All the metadata and hierarchical structure are preserved and opened without any risks.
Explore the verified reviews of IT professionals, enterprise admins, and forensic investigators.
SysInfo OST Viewer 26.6 can quickly open & retrieve the contents of an OST file with/without a specific Outlook or Exchange account. It works with orphaned, minorly corrupted, and oversized OST files and can access them in no time from when the application is opened.
Free to use, fully offline, and completely safe, as demonstrated by others who have used it before.
Step 1: Launch and load the SysInfo OST File Viewer and pick an option to upload OST files in the Dashboard.
Step 2: Click Load/Find to automatically fetch files or Browse to upload them manually. Verify the file status and tap Load to make the final upload.
Step 3: Preview the contents within your OST file to check the integrity preservation and select files to export
Step 4: Click Save Outlook Data File and choose an output format for your OST file. Also, pick the custom filters to apply before the export.
Step 5: Select the destination path for your new file with Browse and tap Save to export OST files.
Step 6: At last, wait until the process completes. Click Open Folder to check output files. OK to close and View Logs to see the preview and export report.
