What is the MSO file format?
A file with the .mso extension contains data related to an email attachment sent with the Microsoft Outlook email client in the form of an Office 2000 or later document, typically generated by Microsoft Word. Information in an .mso file allows Outlook to open the attachment as part of an email message as an inline HTML document.
Internally, .mso files use an ActiveMime binary wrapper. They begin with the literal ASCII text ActiveMime at byte offset 0, followed by a short header and a zlib-compressed payload. Decompressed, the content is a standard OLE2 Compound File holding embedded objects or a VBA project. This structure is distinct from winmail.dat (which uses the TNEF format) even though both can cause "unexpected attachment" problems for non-Outlook recipients.
.mso files are typically named oledata.mso (occasionally editdata.mso) and are invisible to both the sender and receiver when both parties use Outlook for email. If the recipient uses a different email client, the .mso file appears as a standard email attachment, and the document file is also treated as an attachment. A separate file named header.html, which contains the message header, will also be present.
Because .mso files are internal packaging artifacts, they hold no information directly useful to end users and should not be opened manually. Power users can inspect them by decompressing the zlib payload and then treating the result as an OLE2 Compound File using a tool such as 7-Zip.
Security & safety
RISK: MEDIUMA genuine oledata.mso from a known contact is harmless leftover formatting and can be ignored or deleted. However, the ActiveMime/OLE structure can carry VBA macros, and crafted .mso files have been used to deliver Office macro malware, so antivirus engines flag them. Never run or "extract and open" an .mso from an unknown or unexpected sender. The safe response is almost always to leave it alone and read the real message body.
Format details
in a nutshell- Microsoft Organization Chart file - A chart saved by the legacy Microsoft Organization Chart applet, unrelated to the oledata.mso email artifact.
Programs that open MSO files
Windows2 apps
Microsoft Outlook
Paid If the sender uses a matching Outlook version the object renders inline in the message - there is usually nothing to 'open' as a separate file. 
7-Zip
Open-source Advanced/forensic only: opens the inner OLE compound file after the ActiveMime layer is decompressed - not a normal way to recover content. 
macOS1 app
Microsoft Outlook
Paid Same as Windows: the embedded object renders in the message when the format matches; the .mso itself is not meant to be opened standalone. Technical details
deep spec| Developer | Microsoft |
| Format type | ActiveMime-wrapped OLE2 Compound File (binary) |
| File signature (magic bytes) | ASCII text `ActiveMime` at byte offset 0 |
| Internal compression | zlib deflate stream; compressed payload typically begins near byte offset 0x32 |
| Decompressed container | OLE2 Compound File (magic bytes `D0 CF 11 E0 A1 B1 1A E1`) |
| MIME type | `application/x-mso` |
| Typical filenames | `oledata.mso`, `editdata.mso` |
| Generating application | Microsoft Outlook (Office 2000 and later) when sending HTML-formatted email with embedded Office documents |
| Visibility in Outlook | Hidden from both sender and recipient; handled silently as HTML inline packaging |
| Visibility in other clients | Surfaces as a stray email attachment in non-Outlook clients such as Thunderbird or Apple Mail |
| Companion file | `header.html` appears alongside `.mso` in non-Outlook clients and contains the email message header |
| Distinction from winmail.dat | Both are Outlook packaging artifacts, but `.mso` uses ActiveMime/OLE2 while `winmail.dat` uses TNEF (magic bytes `78 9F 3E 22`) |
| End-user utility | Contains no data directly accessible or meaningful to the end user; is a transparent internal container |
| Inspection method | Decompress the zlib payload (e.g., with Python `zlib.decompress()`), then open the result as an OLE2 file using 7-Zip or `oletool` |
| Platform | Generated by Outlook for Windows only; Office for Mac produces different attachment packaging formats |
| Released | circa 1999-2000 (Office 2000 HTML mail/web-page export) |
MSO conversions
Community Q&A
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