.MSG

Introduction

An .msg file is a single item saved from Microsoft Outlook on Windows: usually an email, but it can also be a meeting request, a contact or a task. It is what Outlook writes when a message is dragged out to a folder or saved with “Save As”. Unlike an EML file, which holds the message as it was sent, an .msg file holds Outlook’s own list of the item’s properties, so it keeps details such as flags and categories.

Microsoft publishes the format as [MS-OXMSG] in its Open Specifications.

Structure

The file is a Compound File Binary file, the same container as old Word and Excel files, so it starts with D0 CF 11 E0 A1 B1 1A E1. Inside are storages and streams, like folders and files:

• __properties_version1.0: a table of the item’s fixed-size properties, such as flags, dates and sizes
• __substg1.0_XXXXYYYY: one stream per variable-size property, where XXXX is the property ID and YYYY its type in hexadecimal; for example __substg1.0_0037001F is the subject as UTF-16 text
• __recip_version1.0_#00000000 and so on: one storage per recipient
• __attach_version1.0_#00000000 and so on: one storage per attachment, which can hold another whole message
• __nameid_version1.0: the mapping for named properties

The body may be stored as plain text, as HTML, or as compressed RTF, often more than one of these.

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