Sustainability of Digital Formats: Planning for Library of Congress Collections

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Microsoft Outlook Item (MSG)

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name Microsoft Outlook Item (MSG)

The Outlook Item MSG (.msg) file format is a syntax for storing a single Message object, such as an email, an appointment, a contact, a task, and so on, in a file. Any properties that are present on the Message object, including Attachment objects, are also present in the MSG file.

MSG is based on the CFB_3 format which implements a simplified file system through a hierarchical collection of storage objects and stream objects which behave as directories and files, respectively within a single file. Message files contain objects which contain properties and collections of properties. For all intents and purposes, objects are represented by storages and properties are represented and reside in streams.

MSG specifies five storage elements, each representing one major component of the Message object and a number of streams are contained within those storages, each stream representing a property (or a set of properties) of that component.

The storages are:
  • Recipient object storage
  • Attachment object storage
  • Embedded Message object storage
  • Custom attachment storage
  • Named property mapping storage

The numbers and types of storages and streams present in a MSG file depend on the type of Message object, the number of Recipient objects and Attachment objects it has, and other properties. Properties define attributes of the object like the sender email, whether a read receipt was requested by the sender, whether this message was auto forwarded, an attachment’s filename, etc.

String properties in MSG must be either Unicode or non-Unicode. The .msg File Format does not allow the presence of both simultaneously.

Production phase MSG files provide a mechanism for the storage of an email message, an appointment, a contact, or a task within a file system.
Relationship to other formats
    Defined via CFB_3, Compound File Binary File Format, Version 3

Local use Explanation of format description terms

LC experience or existing holdings The Library of Congress includes MBOX files in its collections, especially in the Manuscripts and Music Divisions as well as other personal papers repositories.
LC preference The Library of Congress Recommended Formats Statement (RFS) lists MSG as an acceptable format for Email: For individual messages.

Sustainability factors Explanation of format description terms

Disclosure Fully documented. Proprietary file format developed by Microsoft.
    Documentation [MS-OXMSG]: Outlook Item (.msg) File Format specification available from Microsoft.
Adoption MSG is implemented in the following Microsoft products: Microsoft Exchange Server 2003-2013 and Microsoft Office Outlook 2003-2013.
    Licensing and patents

The MSG format specification is covered by the Microsoft Interoperability Program. See Useful references below. Microsoft claims no patents in the MSG format. Patents and licenses may apply to some operations and protocols that are used by Microsoft in its electronic mail products and that the MSG format is designed to support. In late 2015, the only patents listed by Microsoft as associated with the related protocol specifications listed in this format description are associated with operational systems for managing messages according to a retention policy: US 8620869 B2 -- Techniques to manage retention policy tags; and US 20140095641 A1 -- Techniques to manage retention policy tags.

Transparency A .msg file can be saved in Outlook or compatible email client and then viewed in an hex editor or binary file parser.

See CFB_3

External dependencies None
Technical protection considerations None

Quality and functionality factors Explanation of format description terms

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension msg
From specification
Internet Media Type application/
Not registered with IANA but listed on MIME Types by Content Type.
File signature   See CFB_3
Pronom PUID x-fmt/430
See for Outlook 97-2003.
Wikidata Title ID Q61707607
See for Outlook 97-2003.

Notes Explanation of format description terms


Microsoft reports that there are scenarios for which storing a Message object in the MSG format would not be advisable:

  • Maintaining a large standalone archive. A better option would be a more full-featured format that can render views more efficiently.
  • Sending information to an unknown receiver. In this scenario, it is possible that the format is not supported by the receiver or that information that is private or irrelevant might be transmitted.

MSG provides some security mechanisms for ensuring that clients read the correct number of bytes from constituent streams.

  • In the case of multiple-valued variable length properties, the length stream contains the lengths of each value. Clients can compare the lengths obtained from there with the actual length of the value streams. If they are not in sync, it can be assumed that there is data corruption.
  • In case of the strings, stream entries are stored prefixed with their lengths; and if any inconsistency is detected, clients can assume that there is data corruption.

Format specifications Explanation of format description terms

Useful references


Last Updated: 03/01/2023