What is a MSG file?
MSG is a proprietary binary file format used by Microsoft Outlook to save individual email messages as files on disk. Each MSG file contains the complete email including the body (plain text, RTF, or HTML), headers, sender and recipient addresses, timestamps, embedded attachments, and MAPI (Messaging Application Programming Interface) metadata properties. MSG files use the Microsoft Compound Document File Format (OLE2/COM Structured Storage), the same underlying container as older .doc and .xls files.
When you drag an email from Outlook to your desktop or use File → Save As, Outlook creates a .msg file. This makes MSG the standard format for archiving individual emails and sharing specific messages outside of an email system.
How to open MSG files
- Microsoft Outlook (Windows, macOS) — Double-click to open; native format
- Mozilla Thunderbird (Windows, macOS, Linux) — With the “MSGViewer” or “ImportExportTools NG” add-on
- MSG Viewer Pro (Windows) — Free dedicated viewer without requiring Outlook
- FreeViewer MSG Viewer (Windows) — Free, no Outlook needed
- eml-viewer.com / msg.ai — Online viewers (avoid for sensitive/confidential messages)
- python-msg — Parse MSG files programmatically
Technical specifications
| Property | Value |
|---|---|
| Container format | OLE2 Compound Document (Microsoft Structured Storage) |
| Body formats supported | Plain text, RTF, HTML (all three may be embedded simultaneously) |
| Attachments | Embedded as OLE sub-streams within the compound document |
| Properties | MAPI properties (PR_SUBJECT, PR_BODY, PR_SENDER_EMAIL_ADDRESS, etc.) |
| Magic bytes | D0 CF 11 E0 (OLE2 signature, shared with .doc/.xls) |
| MIME type | application/vnd.ms-outlook |
| Max attachment size | Limited by Outlook settings (20–150 MB by default per message) |
Common use cases
- Email archiving: Saving important messages from Outlook as individual files on disk or in network shares, outside of the PST/Exchange mailbox
- Legal discovery (eDiscovery): MSG files preserve the original email with all metadata intact, making them suitable for legal evidence and regulatory compliance
- Email migration: Moving individual messages between mail systems or presenting emails outside of a mail client
- Ticketing systems: Helpdesk tools that process email-based support requests sometimes store incoming emails as MSG files
- Backup: Attorneys, auditors, and compliance teams archive MSG files as evidence of business communications
MSG vs EML
| Feature | MSG | EML |
|---|---|---|
| Standard | Microsoft proprietary | RFC 5322 (open) |
| Open without Outlook | Requires viewer tool | Any text editor |
| Full fidelity (RTF, MAPI) | ✅ | Partial |
| Platform | Windows/macOS Outlook | Universal |
| Created by | Outlook drag-to-desktop | Most other email clients |
EML is an open format readable in any text editor or email client. MSG preserves more Outlook-specific metadata but requires proprietary tools to read.
Converting MSG to other formats
# Using msgconvert (Linux — part of Email::Outlook::Message Perl module)
msgconvert message.msg
# Using python-msg (Python library)
pip install extract-msg
python -c "
import extract_msg
msg = extract_msg.Message('email.msg')
print(msg.subject)
print(msg.body)
msg.saveAttachments()
"The extract-msg Python library is the most reliable cross-platform solution for reading MSG files programmatically without Outlook installed.
Security considerations
MSG files can contain embedded HTML with JavaScript (though Outlook’s security settings block active content by default), phishing links, and malicious attachments. MSG files received from unknown sources should be scanned with antivirus before opening. In corporate environments, the Exchange/Outlook trust model evaluates sender reputation — MSG files forwarded outside this trust chain lose those protections. Never open MSG files from unknown senders without verifying authenticity through an independent channel.