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Email & CommLow Risk
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.OST File Extension Reference

The .PST and .OST formats are the Microsoft Outlook personal and offline folder files, identified by the !BDN magic bytes. Files with the .OST extension are categorized under Email & Comm and are widely utilized in modern computing environments for storing, rendering, and exchanging structured data. By verifying the header signature (21 42 44 4E) using forensic byte inspection tools, users can authenticate file integrity and rule out disguised executable payloads.

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Quick Specification & Metadata

.OST
File Extension

.OST

Category

Email & Comm

MIME Type

application/vnd.ms-outlook

Developer / Owner

Microsoft Corporation

First Released

N/A

Licensing Standard

Proprietary / Patent Licensed

Format Structure

Binary Header Data

Typical File Size

100 MB - 50 GB

Common Usage:ost file, open ost, outlook ost file

Online .OST Viewer & Live Inspector

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Client-Side Memory

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What is a .OST File? (Technical Definition & Architecture)

ISO / IETF / Industry Standard

The .PST and .OST formats are the Microsoft Outlook personal and offline folder files, identified by the !BDN magic bytes. Files with the .OST extension are categorized under Email & Comm and are widely utilized in modern computing environments for storing, rendering, and exchanging structured data. By verifying the header signature (21 42 44 4E) using forensic byte inspection tools, users can authenticate file integrity and rule out disguised executable payloads.

Binary Encoding
Structured Data Stream

Formatted byte alignment adhering to international format specifications.

Validation Standard
Specification Conformance

Adheres to formal schema and protocol documentation.

Cross-Platform Portability
Cross-Platform Interoperability

Parsed and executed identically across Windows, macOS, and Linux kernels.

OST Binary Structure & Header Specifications

The .OST format organizes data into validated sequential or hierarchical byte structures. Inspecting the header offset reveals the magic byte sequence 21 42 44 4E.

  • File Signature (Magic Bytes): 21 42 44 4E
  • MIME Content-Type: application/vnd.ms-outlook
  • Payload Structure: Optimized data stream with metadata headers and checksum records.
  • Safety Classification: Low Risk – OST files mirror server mailboxes and can contain malicious attachments Delete the OST to force Outlook to rebuild a corrupted cache Files ending in .OST should always be inspected for valid magic bytes to verify they match legitimate Email & Comm specifications.

How to Open .OST Files on Windows, Mac & Android

Step-by-Step OS Guide
How to Open on Windows 10/11
  1. 1Primary Application: Double-click or open directly with Microsoft Outlook.
  2. 2Alternative Software: Open using SysTools OST Viewer.
  3. 3File Association: Right-click the file, select Open with > Choose another app, and check Always use this app.
How to Open on Mac & iPhone
  1. 1Native macOS Application: Open with Microsoft Outlook.
  2. 2Quick Look Preview: Highlight the file in Finder and tap the Spacebar.
  3. 3Set Default App: Press Cmd + I, expand Open with, select your application, and click Change All.
How to Open on Android & Mobile
  1. 1Compatible Mobile Apps: Download a dedicated viewer for .ost from the Google Play Store.
  2. 2Cloud Storage: Open through Google Drive, Dropbox, or OneDrive mobile apps.
  3. 3Online Browser Conversion: Use AnyFileX mobile browser tools to convert the file into universal PDF or image formats.

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.OST to .JPGUniversal ExportStandard Output

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.OST MIME Types & HTTP Server Headers

IANA Standard

When serving .ost files over HTTP or configuring REST APIs, web servers must send the official IANA registered Content-Type headers:

MIME Type IdentifierStandard UsageFile ExtensionsCopy
application/vnd.ms-outlookStandard official MIME type for .ost files.ost
Nginx / Apache Server Configuration
# Nginx mime.types
types {
    application/vnd.ms-outlook             ost;
}

# Apache .htaccess
AddType application/vnd.ms-outlook .ost

.OST Magic Bytes & Hex File Signatures

Offset 0x00

In digital forensics and file signature analysis, authentic .ost files begin with a standardized magic byte sequence at byte offset 0x00 to verify file integrity and prevent extension spoofing:

Standard Hex Header Signature
Hex Representation:21 42 44 4E
ASCII Representation:OST
Byte OffsetOffset 0x00
Leading Hex Signature21 42 44 4E
ASCII RepresentationOST
Forensic DomainEmail & Comm File Format

.OST vs .PDF (Detailed Format Comparison)

Head-to-Head
Feature Metric.OST.PDFAdvantage
Format Full NameOST File FormatPortable Document FormatTie / Equal
CategoryEmail & CommDocumentsTie / Equal
MIME Content-Typeapplication/vnd.ms-outlookapplication/pdfTie / Equal
Binary Header (Magic Bytes)21 42 44 4E25 50 44 46 2D (%PDF-)Tie / Equal
Typical Storage Footprint100 MB - 50 GB200 KB – 25 MBTie / Equal
Universal Native SupportRequires SoftwareHigh / OS StandardTie / Equal

.OST Compatibility Matrix

Windows 11 / 10

Supported natively or through Microsoft Outlook

Full Support
macOS Sonoma / Ventura

Supported natively or via SysTools OST Viewer

Supported
Linux (Ubuntu / Fedora)

Supported via open-source tools and command line utilities

Supported
iOS & iPadOS

Accessible through compatible mobile viewers or cloud previewers

Mobile App
Android 10+

Supported through dedicated mobile apps or cloud viewers

Mobile App
ChromeOS

Accessible via web browser applications and Chrome extensions

Web / App

Recommended Software for .OST

2 Verified Apps
M

Microsoft Outlook

Microsoft Corporation

Paid / Commercial
4.7 / 5.0
•
Compatible with .OST
S

SysTools OST Viewer

Microsoft Corporation

Paid / Commercial
4.8 / 5.0
•
Compatible with .OST

Common Errors & Troubleshooting .OST Files

Troubleshooting
1Inspect File Header: Use the AnyFileX Magic Byte Detector to verify that the first bytes match 21 42 44 4E.
2Prevent Extension Spoofing: Ensure "File name extensions" is checked in your OS file manager to detect disguised double extensions (e.g., file.OST.exe).
3Verify Checksum Integrity: Calculate SHA-256 or MD5 hashes against source downloads using our online Checksum Verifier.
4Inspect Container Structure: If OST files fail to launch, verify whether the file was truncated during download or corrupted in transit.

Security & Malware Risk Assessment

Low Risk

Can a .OST file carry viruses or malware?

OST files mirror server mailboxes and can contain malicious attachments Delete the OST to force Outlook to rebuild a corrupted cache Files ending in .OST should always be inspected for valid magic bytes to verify they match legitimate Email & Comm specifications.

Executable Macros

No active macro execution support

Sandboxed Parsing

Safe to inspect with AnyFileX's browser sandbox

Safety Checklist Before Opening
  • •Verify file extension in OS File Explorer options ('Show known file extensions').
  • •Verify magic bytes (21 42 44 4E) to ensure the file isn't a renamed executable.
  • •Do not open .OST attachments received unexpectedly via email or instant messenger.

Frequently Asked Questions

A .OST file is a OST File Format associated with Email & Comm. OST (Offline Storage Table) is the cached copy of a Microsoft Exchange or IMAP mailbox that lets Outlook work offline. Like PST it uses the !BDN signature, and it resynchronizes with the server when reconnected.
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    Related Questions

    A .OST file is a OST File Format file format categorized under Email & Comm. OST (Offline Storage Table) is the cached copy of a Microsoft Exchange or IMAP mailbox that lets Outlook work offline. Like PST it uses the !BDN signature, and it resynchronizes with the server when reconnected.

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