What Is an MSG File?
An MSG file is Microsoft Outlook's proprietary format for storing a single email message, calendar item, or contact. Unlike simple text-based formats, MSG files use Microsoft's Compound File Binary (CFB) structure — the same underlying technology used by older `.doc` and `.xls` files.
This means an MSG file is essentially a mini file system containing multiple data streams: the email body (in both plain text and HTML), all header metadata (sender, recipients, dates, subject), and references to any embedded attachments.
Why Can't Windows Open MSG Files Natively?
Despite being a Microsoft format, Windows has never shipped with a built-in MSG file viewer. The operating system recognises the `.msg` extension but has no default handler for it. Without Outlook installed, double-clicking an MSG file will prompt the "How do you want to open this file?" dialog.
This is because parsing the CFB binary structure requires specialised logic that goes beyond simple file reading. The email body, headers, and attachment metadata are stored in separate OLE streams within the file, each requiring correct interpretation.
What Data Is Stored Inside?
A typical MSG file contains:
- Sender information — display name and email address
- Recipients — To, CC, and BCC fields with names and addresses
- Subject line and timestamps (sent date, received date)
- Email body — often in both plain text and rich HTML with embedded CSS
- Embedded images — inline images referenced by the HTML body
- Attachment metadata — file names, sizes, and MIME types of all attached files
How OpenMSG Handles MSG Files
OpenMSG is purpose-built to parse the CFB binary structure of MSG files and present all of this information in a clean, readable interface — without requiring Outlook or any Microsoft Office installation.
When you open an MSG file in OpenMSG, it reads each OLE stream, reconstructs the HTML email body with its original formatting and embedded images, and displays all metadata and attachment information clearly.
Because OpenMSG processes everything locally on your Windows PC, your email data never leaves your machine.
Conclusion
MSG files are more complex than they appear. They are full binary containers that require specialised parsing to read correctly. If you regularly receive MSG files and don't have Outlook, a lightweight dedicated viewer like OpenMSG is the simplest and most secure way to access their contents.