What is the MSG file format?
Files with the .msg extension are used by Microsoft Outlook to save email messages. The program stores all the most important information regarding an email message in a .msg file, including the subject, body content, sender and receiver data, date, hyperlinks, and attachments.
When is a .msg file created?
Outlook generates a .msg file whenever an email message is being saved or created in the program. However, the file can also be generated in other circumstances - not only for email messages, but also for other special items in Outlook such as calendar appointments, contacts, or tasks. A .msg file is created whenever one of those items is saved to disk or exported from the application.
Opening .msg files
.msg files are not stored in a simple text form but in a binary format - specifically, the Microsoft Compound File Binary (OLE2 structured storage) container, the same underlying format used by legacy .doc and .xls documents. Originally, only applications implementing the MAPI interface (Microsoft's Messaging Applications Programming Interface) could open them reliably, which is why importing .msg files outside Outlook has historically been a bit difficult. Today a wider range of tools supports the format, including dedicated viewers and the msgconvert command-line utility on Linux. .msg files can also be converted to the more portable .eml format for use with other mail clients. Exporting .msg files remains common mainly in order to archive them, store them, or scan embedded attachments with antivirus programs. Related Outlook data stores include .pst files, which aggregate many messages in a single archive.
Security & safety
RISK: MEDIUMA .msg can carry the same threats as any email: malicious attachments, phishing links and tracking pixels. Because attachments are embedded as binary streams, open an unexpected .msg in a proper viewer (which decodes safely) rather than trying to extract files blindly, and don't open the attachments or click links unless you trust the sender. Two extra notes: .msg is binary OLE storage, so renaming it to .txt won't reveal readable content; and when using ONLINE .msg viewers, remember the file is uploaded to a third-party server - don't use them for confidential or sensitive mail.
Format details
in a nutshell- Sound Designer / MIDI message file (.msg) - A rare audio/MIDI data file unrelated to Outlook saved messages.
Programs that open MSG files
Windows4 apps
Microsoft Outlook
Paid Double-click the .msg to open it as a message; save attachments from within. The native app. 
OpenMSG (Microsoft Store)
Free Lightweight Store app made to open .msg without Outlook; install and double-click. Shows body, headers and attachments.
Mozilla Thunderbird + ImportExportTools NG
Open-source Free cross-platform mail client; with the ImportExportTools NG add-on it imports/opens .msg files. Good no-Outlook option.
MsgViewer Pro (Encryptomatic)
Paid Dedicated .msg/.eml viewer with batch and conversion features; the free online viewer covers single files. 
macOS2 apps
Klammer
Paid Dedicated .msg/.eml viewer for Mac; double-click or open the .msg to read body and attachments properly.
Mozilla Thunderbird + ImportExportTools NG
Open-source Install Thunderbird and the ImportExportTools NG add-on to import/read .msg files on macOS. 
Linux2 apps
Mozilla Thunderbird + ImportExportTools NG
Open-source Use the add-on to import/read .msg; or convert with msgconvert (libemail-outlook-message-perl) to .eml first.
msgconvert (Email::Outlook::Message)
Open-source Command line: 'msgconvert file.msg' produces a readable .eml that any Linux mail client opens.
Klammer
Paid Open the .msg attachment with Klammer on iPhone/iPad to view body and attachments. iOS Mail can't render .msg properly.
Encryptomatic / CoolUtils online MSG viewer
Free Upload the .msg in a browser to read it instantly without installing anything (avoid for confidential mail - it's uploaded to a server). Technical details
deep spec| Container format | Microsoft Compound File Binary (OLE2 structured storage) - the same CFB container used by legacy `.doc` and `.xls` files |
| MIME type | `application/vnd.ms-outlook` |
| File signature (magic bytes) | `D0 CF 11 E0 A1 B1 1A E1` at byte offset 0 - the OLE2 CFB header, shared by all Compound File documents |
| Internal layout | Message properties, body text, and attachments stored as individually named streams and sub-storages inside the CFB container |
| Body format options | Stores plain text (`PR_BODY`), HTML (`PR_HTML`), and compressed RTF (`PR_RTF_COMPRESSED`) simultaneously; the client renders the richest available version |
| String encoding | Text properties stored as Unicode (UTF-16LE); older messages may also carry an ANSI/Windows-1252 fallback copy |
| Attachment storage | Each attachment occupies a dedicated sub-storage (`__attach_version1.0_#XXXXXXXX`) holding its own binary data and MAPI property streams |
| Property tag scheme | Each MAPI property uses a 32-bit tag: upper 16 bits = property ID, lower 16 bits = type code (e.g., `PT_UNICODE`, `PT_BINARY`, `PT_SYSTIME`) |
| Supported Outlook item types | Emails (`IPM.Note`), contacts (`IPM.Contact`), tasks (`IPM.Task`), appointments (`IPM.Appointment`), sticky notes (`IPM.StickyNote`), and distribution lists |
| Embedded message nesting | Forwarded or attached emails stored as nested `.msg` sub-storages within the parent file, to arbitrary depth |
| Preserved routing metadata | Sender, all recipient classes (To/Cc/Bcc), delivery timestamps, read/unread flag, importance level, sensitivity, and conversation thread ID |
| Protocol specification | Publicly documented by Microsoft as the [MS-OXMSG] open protocol; enables third-party libraries such as `libpff` and `msg-extractor` |
| Developer | Microsoft Corporation |
| Typical file size | A few kilobytes for text-only messages; can reach hundreds of megabytes when large files are embedded as attachments |
| Released | Late 1990s with Microsoft Outlook; format documented publicly by Microsoft as [MS-OXMSG] |
| Specification | learn.microsoft.com |
MSG conversions
Community Q&A
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