[MS-OXMSG]:
Outlook Item (.msg) File Format
Intellectual Property Rights Notice for Open Specifications Documentation


Technical Documentation. Microsoft publishes Open Specifications documentation for
protocols, file formats, languages, standards as well as overviews of the interaction among each
of these technologies.



Copyrights. This documentation is covered by Microsoft copyrights. Regardless of any other
terms that are contained in the terms of use for the Microsoft website that hosts this
documentation, you may make copies of it in order to develop implementations of the
technologies described in the Open Specifications and may distribute portions of it in your
implementations using these technologies or your documentation as necessary to properly
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modification, any schema, IDL’s, or code samples that are included in the documentation. This
permission also applies to any documents that are referenced in the Open Specifications.

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

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described in the Open Specifications. Neither this notice nor Microsoft's delivery of the
documentation grants any licenses under those or any other Microsoft patents. However, a given
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Promise. If you would prefer a written license, or if the technologies described in the Open
Specifications are not covered by the Open Specifications Promise or Community Promise, as
applicable, patent licenses are available by contacting iplg@microsoft.com.

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www.microsoft.com/trademarks.



Fictitious Names. The example companies, organizations, products, domain names, e-mail
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association with any real company, organization, product, domain name, email address, logo,
person, place, or event is intended or should be inferred.

Reservation of Rights. All other rights are reserved, and this notice does not grant any rights
other than specifically described above, whether by implication, estoppel, or otherwise.
Tools. The Open Specifications do not require the use of Microsoft programming tools or
programming environments in order for you to develop an implementation. If you have access to
Microsoft programming tools and environments you are free to take advantage of them. Certain
Open Specifications are intended for use in conjunction with publicly available standard
specifications and network programming art, and assumes that the reader either is familiar with the
aforementioned material or has immediate access to it.

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Revision Summary
Date

Revision
History

Revision
Class

04/04/2008

0.1

Initial Availability.

04/25/2008

0.2

Revised and updated property names and other
technical content.

06/27/2008

1.0

Initial Release.

08/06/2008

1.01

Revised and edited technical content.

09/03/2008

1.02

Updated references.

12/03/2008

1.03

Updated IP notice.

04/10/2009

2.0

Updated technical content for new product releases.

07/15/2009

3.0

Major

Revised and edited for technical content.

11/04/2009

3.1.0

Minor

Updated the technical content.

02/10/2010

4.0.0

Major

Updated and revised the technical content.

05/05/2010

4.0.1

Editorial

Revised and edited the technical content.

08/04/2010

5.0

Major

Significantly changed the technical content.

11/03/2010

5.0.1

Editorial

Changed language and formatting in the technical
content.

03/18/2011

6.0

Major

Significantly changed the technical content.

08/05/2011

6.0

No change

No changes to the meaning, language, or formatting of
the technical content.

10/07/2011

6.0

No change

No changes to the meaning, language, or formatting of
the technical content.

01/20/2012

7.0

Major

Significantly changed the technical content.

04/27/2012

7.1

Minor

Clarified the meaning of the technical content.

07/16/2012

8.0

Major

Significantly changed the technical content.

10/08/2012

9.0

Major

Significantly changed the technical content.

02/11/2013

9.0

No change

No changes to the meaning, language, or formatting of
the technical content.

Comments

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Table of Contents
1

Introduction ............................................................................................................. 5
1.1 Glossary ............................................................................................................... 5
1.2 References ............................................................................................................ 6
1.2.1 Normative References ....................................................................................... 6
1.2.2 Informative References ..................................................................................... 6
1.3 Overview .............................................................................................................. 6
1.3.1 Compound Files ............................................................................................... 7
1.3.2 Properties........................................................................................................ 7
1.3.3 Storages ......................................................................................................... 7
1.3.4 Top Level Structure .......................................................................................... 7
1.4 Relationship to Protocols and Other Structures .......................................................... 7
1.5 Applicability Statement ........................................................................................... 8
1.6 Versioning and Localization ..................................................................................... 8
1.7 Vendor-Extensible Fields ......................................................................................... 8

2

Structures ................................................................................................................ 9
2.1 Properties ............................................................................................................. 9
2.1.1 Properties of a .msg File .................................................................................... 9
2.1.1.1 PidTagStoreSupportMask ............................................................................. 9
2.1.1.2 Other Properties ......................................................................................... 9
2.1.2 Fixed Length Properties ..................................................................................... 9
2.1.3 Variable Length Properties ............................................................................... 10
2.1.4 Multiple-Valued Properties ............................................................................... 10
2.1.4.1 Fixed Length Multiple-Valued Properties ...................................................... 11
2.1.4.2 Variable Length Multiple-Valued Properties .................................................. 11
2.1.4.2.1 Length Stream .................................................................................... 12
2.1.4.2.1.1 Length for PtypMultipleBinary.......................................................... 12
2.1.4.2.1.2 Length for PtypMultipleString8 or PtypMultipleString .......................... 12
2.1.4.2.2 Value Streams .................................................................................... 13
2.2 Storages ............................................................................................................. 13
2.2.1 Recipient Object Storage ................................................................................. 13
2.2.2 Attachment Object Storage ............................................................................. 14
2.2.2.1 Embedded Message Object Storage ............................................................ 14
2.2.2.2 Custom Attachment Storage ...................................................................... 15
2.2.3 Named Property Mapping Storage .................................................................... 15
2.2.3.1 Property ID to Property Name Mapping ....................................................... 15
2.2.3.1.1 GUID Stream ...................................................................................... 15
2.2.3.1.2 Entry Stream ...................................................................................... 16
2.2.3.1.2.1 Index and Kind Information ............................................................ 16
2.2.3.1.3 String Stream ..................................................................................... 17
2.2.3.2 Property Name to Property ID Mapping Streams ........................................... 17
2.2.3.2.1 Determining GUID Index ...................................................................... 18
2.2.3.2.2 Generating Stream ID .......................................................................... 18
2.2.3.2.2.1 Stream ID Equation ....................................................................... 18
2.2.3.2.3 Generating Stream Name ..................................................................... 18
2.2.3.2.4 Obtaining Stream Data ........................................................................ 19
2.3 Top Level Structure .............................................................................................. 19
2.4 Property Stream .................................................................................................. 20
2.4.1 Header .......................................................................................................... 20
2.4.1.1 Top Level................................................................................................. 20

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2.4.1.2 Embedded Message object Storage ............................................................. 21
2.4.1.3 Attachment Object Storage or Recipient Object Storage ................................ 22
2.4.2 Data ............................................................................................................. 22
2.4.2.1 Fixed Length Property Entry ....................................................................... 22
2.4.2.1.1 Fixed Length Property Value ................................................................. 23
2.4.2.2 Variable Length Property or Multiple-Valued Property Entry............................ 23
3

Structure Examples ................................................................................................ 25
3.1 From Message Object to .msg File.......................................................................... 25
3.2 Named Property Mapping ...................................................................................... 28
3.2.1 Property ID to Property Name .......................................................................... 28
3.2.1.1 Fetching the Name Identifier ...................................................................... 28
3.2.1.1.1 Numerical Named Property ................................................................... 28
3.2.1.1.2 String Named Property ........................................................................ 29
3.2.1.2 Fetching the GUID .................................................................................... 29
3.2.2 Property Name to Property ID .......................................................................... 30
3.3 Custom Attachment Storage ................................................................................. 31

4

Security .................................................................................................................. 33
4.1 Security Considerations for Implementers ............................................................... 33
4.2 Index of Security Parameters ................................................................................ 33

5

Appendix A: Product Behavior ................................................................................ 34

6

Change Tracking..................................................................................................... 35

7

Index ..................................................................................................................... 36

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1

Introduction
The Outlook Item (.msg) File Format is used to format a Message object, such as an e-mail
message, an appointment, a contact, a task, and so on, for storage in the file system.
Sections 1.7 and 2 of this specification are normative and can contain the terms MAY, SHOULD,
MUST, MUST NOT, and SHOULD NOT as defined in RFC 2119. All other sections and examples in this
specification are informative.

1.1

Glossary

The following terms are defined in [MS-GLOS]:
cyclic redundancy check (CRC)
GUID
little-endian
property set
Unicode
The following terms are defined in [MS-OXGLOS]:
Attachment object
Embedded Message object
Message object
named property
non-Unicode
property ID
property name
property tag
property type
Recipient object
storage
store
stream
tagged property
The following terms are specific to this document:
name identifier: The identifier that is used to refer to a named property. It can be either a
LONG numerical value or a Unicode string. It is represented by the Kind member variable of
the PropertyName structure, depending on the value of the Kind member variable.
named property mapping: A process that converts PropertyName structures to property IDs
and vice-versa. Named properties can be referred to by their PropertyName. However, before
accessing the property on a specific store, named properties need to be mapped to property
IDs that are valid for that store. The reverse is also true. When properties need to be copied
across stores, property IDs that are valid for the source store need to be mapped to their
PropertyName structures before they can be sent to the destination store.
numerical named property: A named property that has a numerical name identifier, which is
stored in the LID field of a PropertyName structure.
string named property: A named property that has a Unicode string as a name identifier, which
is stored in the Name field of a PropertyName structure. A string named property can have
any property type; "string" refers only to its name identifier.

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MAY, SHOULD, MUST, SHOULD NOT, MUST NOT: These terms (in all caps) are used as
described in [RFC2119]. All statements of optional behavior use either MAY, SHOULD, or
SHOULD NOT.

1.2

References

References to Microsoft Open Specifications documentation do not include a publishing year because
links are to the latest version of the technical documents, which are updated frequently. References
to other documents include a publishing year when one is available.

1.2.1

Normative References

We conduct frequent surveys of the normative references to assure their continued availability. If
you have any issue with finding a normative reference, please contact dochelp@microsoft.com. We
will assist you in finding the relevant information. Please check the archive site,
http://msdn2.microsoft.com/en-us/library/E4BD6494-06AD-4aed-9823-445E921C9624, as an
additional source.
[MS-CFB] Microsoft Corporation, "Compound File Binary File Format".
[MS-DTYP] Microsoft Corporation, "Windows Data Types".
[MS-OXCDATA] Microsoft Corporation, "Data Structures".
[MS-OXCMSG] Microsoft Corporation, "Message and Attachment Object Protocol".
[MS-OXPROPS] Microsoft Corporation, "Exchange Server Protocols Master Property List".
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC
2119, March 1997, http://www.rfc-editor.org/rfc/rfc2119.txt

1.2.2

Informative References

[MSDN-STS] Microsoft Corporation, "About Structured Storage", http://msdn.microsoft.com/enus/library/aa378734.aspx
[MS-GLOS] Microsoft Corporation, "Windows Protocols Master Glossary".
[MS-OXGLOS] Microsoft Corporation, "Exchange Server Protocols Master Glossary".
[MS-OXPROTO] Microsoft Corporation, "Exchange Server Protocols System Overview".
[X25] ITU-T, "X25: Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating
Equipment (DCE) for terminals operating in the packet mode and connected to public data networks
by dedicated circuit", ITU-T Recommendation X.25, October 1996, http://www.itu.int/rec/T-RECX.25-199610-I/en

1.3

Overview

The Outlook Item (.msg) File Format is a syntax for storing a Message object, such as an e-mail, an
appointment, a contact, a task, and so on, in a file. Any properties that are present on the Message
object are also present in the .msg file.
For information about a Message object and its properties, see the Message and Attachment Object
Protocol, which is described in [MS-OXCMSG].

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1.3.1

Compound Files

The .msg File Format is based on the Compound File Binary File Format, which is described in [MSCFB]. The paradigm provides for the concept of storages and streams (1), which are similar to
directories and files, except that the entire hierarchy of storages and streams are packaged into a
single file, called a compound file. This facility allows applications to store complex, structured data
in a single file. For more information regarding structured storage in a compound file, see [MSDNSTS].
The format specifies a number of storages, each representing one major component of the Message
object. A number of streams (1) are contained within those storages, each stream representing a
property (or a set of properties) of that component. Nesting is possible, as described by [MS-CFB],
where one storage can contain substorages.

1.3.2

Properties

Properties are stored in streams (1) contained within storages or at the top level of the .msg file.
They can be classified into the following broad categories:
Fixed length properties — For more information, see section 2.1.2.
Variable length properties — For more information, see section 2.1.3.
Multiple-valued properties — For more information, see section 2.1.4.
Regardless of the category, a property is either a tagged property or a named property. There is
no difference in the way the property is stored based on that attribute. However, for all named
properties, appropriate mapping information has to be provided as specified by the named property
mapping storage.

1.3.3

Storages

Storages are used to represent major components of the Message object. The .msg File Format
defines the following storages:
Recipient object storage — For more information, see section 2.2.1.
Attachment object storage — For more information, see section 2.2.2.
Embedded Message object storage — For more information, see section 2.2.2.1.
Custom attachment storage — For more information, see section 2.2.2.2.
Named property mapping storage — For more information, see section 2.2.3.

1.3.4

Top Level Structure

The top level of the .msg file represents the entire Message object. Depending on what type of
Message object it is, the number of Recipient objects and Attachment objects it has, and the
properties that are set on it, there can be different storages and streams in the corresponding .msg
file.

1.4

Relationship to Protocols and Other Structures

The .msg File Format has the following relationships to protocols and other structures:

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It is based on the Compound File Binary File Format, as described in [MS-CFB].
It uses structures and data types that are described in [MS-OXCDATA] and [MS-DTYP].
It uses the properties that are used by the Message and Attachment Object Protocol, as
described in [MS-OXCMSG].
For conceptual background information and overviews of the relationships and interactions between
this and other protocols, see [MS-OXPROTO].

1.5

Applicability Statement

The .msg File Format is used to store a Message object in a .msg file, which then can be shared
between clients or stores that use the file system.
There are scenarios for which storing a Message object in the .msg File Format would not be
particularly well-suited. For example, a .msg file is not suitable in the following scenarios:
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.

1.6

Versioning and Localization

None.

1.7

Vendor-Extensible Fields

The .msg File Format does not provide any extensibility beyond what is specified in [MS-CFB].

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2

Structures

2.1

Properties

Properties are stored in streams (1) contained within one of the storages or at the top level of the
.msg file. There is no difference in property storage semantics for named properties when compared
to tagged properties.

2.1.1

Properties of a .msg File

2.1.1.1

PidTagStoreSupportMask

Type: PtypInteger32 ([MS-OXCDATA] section 2.11.1)
The PidTagStoreSupportMask property ([MS-OXPROPS] section 2.1100) indicates whether string
properties within the .msg file are Unicode-encoded or not. This property defines multiple flags, but
only the STORE_UNICODE_OK flag is valid. All other bits MUST be ignored. The settings for this
property are summarized in the following table.
Flag name

Value

Description

STORE_UNICODE_OK

0x00040000

Set if the string properties are Unicode-encoded.

other flags

All values except 0x00040000

All other bits MUST be ignored.

2.1.1.2

Other Properties

A .msg file includes all properties that are present on the Message object that is being stored. If the
Message object includes any Attachment objects, the properties of each Attachment object are also
present in the .msg file. For details about the properties of these objects, see [MS-OXCMSG] section
2.2.1 and section 2.2.2.

2.1.2

Fixed Length Properties

Fixed length properties, within the context of this document, are defined as properties that, as a
result of their type, always have values of the same length.
Following is a list of fixed length property types. All of these property types are specified in [MSOXCDATA] section 2.11.1.
PtypInteger16
PtypInteger32
PtypFloating32
PtypFloating64
PtypBoolean
PtypCurrency
PtypFloatingTime
PtypTime

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PtypInteger64
PtypErrorCode
All fixed length properties are stored in the property stream. Each fixed length property has one
entry in the property stream, and that entry includes its property tag, its value, and a flag
providing additional information about the property.

2.1.3

Variable Length Properties

A variable length property, within the context of this document, is defined as one where each
instance of the property can have a value of a different size. Such properties are specified along
with their lengths or have alternate mechanisms (such as terminating null characters) for
determining their size.
Following is an exhaustive list of property types that are either variable length or stored in a stream
like variable length property types. These property types are specified in [MS-OXCDATA] section
2.11.1.
PtypString
PtypBinary
PtypString8
PtypGuid
PtypObject
Each variable length property has an entry in the property stream. However, the entry contains only
the property tag, the size, and a flag providing more information about the property. The entry does
not contain the variable length property's value. Since the value can be variable in length, it is
stored in an individual stream (1) by itself. Properties of type PtypGuid do not have variable length
values (they are always 16 bytes long). However, like variable length properties, they are stored in
a stream (1) by themselves in the .msg file because the values have a large size. Therefore, they
are grouped along with variable length properties.
The name of the stream where the value of a particular variable length property is stored is
determined by its property tag. The stream name is created by prefixing a string containing the
hexadecimal representation of the property tag with the string "__substg1.0_". For example, if the
property is PidTagSubject ([MS-OXPROPS] section 2.1104), the name of the stream is
"__substg1.0_0037001F", where "0037001F" is the hexadecimal representation of the property tag
for PidTagSubject.
If the PidTagStoreSupportMask property (section 2.1.1.1) is present and has the
STORE_UNICODE_OK (bitmask 0x00040000) flag set, all string properties in the .msg file MUST
be present in Unicode format. If the PidTagStoreSupportMask is not available in the property
stream or if the STORE_UNICODE_OK flag is not set, the .msg file is considered to be nonUnicode and all string properties in the file MUST be in non-Unicode format.
All string properties for a Message object MUST be either Unicode or non-Unicode. The .msg File
Format does not allow the presence of both simultaneously.

2.1.4

Multiple-Valued Properties

A multiple-valued property can have multiple values corresponding to it, stored in an array. All
values of the property MUST have the same type.
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Each multiple-valued property has an entry in the property stream. However, the entry contains
only the property tag, size, and a flag providing more information about the property and not its
value.
The value is stored differently depending upon whether the property is a fixed length multiplevalued property, as specified in section 2.1.4.1, or a variable length multiple-valued property, as
specified in section 2.1.4.2.

2.1.4.1

Fixed Length Multiple-Valued Properties

A fixed length multiple-valued property, within the context of this document, is defined as a
property that can have multiple values, where each value is of the same fixed length type. The
following table is an exhaustive list of fixed length multiple-valued property types and the
corresponding value types. All of the property types and value types in the following table are
specified in [MS-OXCDATA] section 2.11.1.
Property type

Value type

PtypMultipleInteger16

PtypInteger16

PtypMultipleInteger32

PtypInteger32

PtypMultipleFloating32

PtypFloating32

PtypMultipleFloating64

PtypFloating64

PtypMultipleCurrency

PtypCurrency

PtypMultipleFloatingTime

PtypFloatingTime

PtypMultipleTime

PtypTime

PtypMultipleGuid

PtypGuid

PtypMultipleInteger64

PtypInteger64

The array of values of a fixed length multiple-valued property is stored in one stream. The name of
that stream is determined by the property's property tag. The stream name is created by prefixing a
string containing the hexadecimal representation of the property tag with the string "__substg1.0_".
For example, if the property is PidTagScheduleInfoMonthsBusy ([MS-OXPROPS] section 2.1054),
the name of the stream is "__substg1.0_68531003", where "68531003" is the hexadecimal
representation of the property tag for PidTagScheduleInfoMonthsBusy.
The values associated with the fixed length multiple-valued property are stored in the stream
contiguously like an array.

2.1.4.2

Variable Length Multiple-Valued Properties

A variable length multiple-valued property, within the context of this document, is defined as a
property that can have multiple values, where each value is of the same type but can have different
lengths. The following table is an exhaustive list of variable length multiple-valued property types
and the corresponding value types. All of the property types and value types in the following table
are specified in [MS-OXCDATA] section 2.11.1.

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Property type

Value type

PtypMultipleBinary

PtypBinary

PtypMultipleString8

PtypString8

PtypMultipleString

PtypString

For each variable length multiple-valued property, if there are N values, there MUST be N + 1
streams: N streams to store each individual value and one stream to store the lengths of all the
individual values.

2.1.4.2.1

Length Stream

The name of the stream that stores the lengths of all values is derived by prefixing a string
containing the hexadecimal representation of the property tag with the string "__substg1.0_". For
example, if the property is PidTagScheduleInfoDelegateNames ([MS-OXPROPS] section 2.1041),
the stream's name is "__substg1.0_6844101F", where "6844101F" is the hexadecimal
representation of the property tag for PidTagScheduleInfoDelegateNames.
The number of entries in the length stream MUST be equal to the number of values of the multiplevalued property. The entries in the length stream are stored contiguously. The first entry in the
length stream specifies the size of the first value of the multiple-valued property; the second entry
specifies the size of the second value, and so on. The format of length stream entries depends on
the property's type. The following sections specify the format of one entry in the length stream.

2.1.4.2.1.1

Length for PtypMultipleBinary

Each entry in the length stream for a PtypMultipleBinary property ([MS-OXCDATA] section 2.11.1)
has the following structure.

0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

3
0

9

1

Length
Reserved

Length (4 bytes): The length, in bytes, of the corresponding value of the PtypBinary property
([MS-OXCDATA] section 2.11.1).
Reserved (4 bytes): This field MUST be set to 0 when writing a .msg file and MUST be ignored
when reading a .msg file.

2.1.4.2.1.2

Length for PtypMultipleString8 or PtypMultipleString

Each entry in the length stream for a PtypMultipleString8 property or a PtypMultipleString
property ([MS-OXCDATA] section 2.11.1) has the following structure.

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0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

1

Length

Length (4 bytes): The length, in bytes, of the corresponding value of the PtypString8 property
or the PtypString property ([MS-OXCDATA] section 2.11.1). The length includes the NULL
terminating character.

2.1.4.2.2

Value Streams

Each value of the property MUST be stored in an individual stream. The name of the stream is
constructed as follows:
1. Concatenate a string containing the hexadecimal representation of the property tag to the string
"__substg1.0_".
2. Concatenate the character "-" to the result of step 1.
3. Concatenate a string containing the hexadecimal representation of the index of the value within
that property, to the result of step 2. The index used MUST match the index of the value's length,
which is stored in the length stream. The indexes are zero-based.
For example, the first value of the property PidTagScheduleInfoDelegateNames ([MS-OXPROPS]
section 2.1041) is stored in a stream with name "__substg1.0_6844101F-00000000", where
"6844101F" is the hexadecimal representation of the property tag and "00000000" represents the
index of the first value. The second value of the property is stored in a stream with name
"__substg1.0_6844101F-00000001", and so on.
In case of multiple-valued properties of type PtypMultipleString and PtypMultipleString8 ([MSOXCDATA] section 2.11.1), all values of the property MUST end with the NULL terminating
character.

2.2

Storages

2.2.1

Recipient Object Storage

The Recipient object storage contains streams (1) and substorages that store properties pertaining
to one Recipient object.
The following MUST be true for Recipient object storages:
The Recipient object storage representing the first Recipient object is named
"__recip_version1.0_#00000000". The storage representing the second is named
"__recip_version1.0_#00000001" and so on. The digit suffix is in hexadecimal. For example, the
storage name for the eleventh Recipient object is "__recip_version1.0_#0000000A".
A .msg file can have a maximum of 2048 Recipient object storages.
There is exactly one property stream, and it contains entries for all properties of the Recipient
object.
There is exactly one stream for each variable length property of the Recipient object, as specified
in section 2.1.3.

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2.2.2

Attachment Object Storage

The Attachment object storage contains streams (1) and substorages that store properties
pertaining to one Attachment object.
The following MUST be true for Attachment object storages:
The Attachment object storage representing the first Attachment object is named
"__attach_version1.0_#00000000". The storage representing the second is named
"__attach_version1.0_#00000001" and so on. The digit suffix is in hexadecimal. For example,
the storage name for the eleventh Attachment object is "__attach_version1.0_#0000000A"
A .msg file can have a maximum of 2048 Attachment object storages.
There is exactly one property stream, and it contains entries for all properties of the Attachment
object.
There is exactly one stream for each variable length property of the Attachment object, as
specified in section 2.1.3.
There is exactly one stream for each fixed length multiple-valued property of the Attachment
object, as specified in section 2.1.4.1.
For each variable length multiple-valued property of the Attachment object, if there are N values,
there are N + 1 streams, as specified in section 2.1.4.2.
If the Attachment object itself is a Message object, there is an Embedded Message object storage
under the Attachment object storage.
If the Attachment object has a value of afStorage (0x00000006) for the PidTagAttachMethod
property ([MS-OXCMSG] section 2.2.2.9), then there is a custom attachment storage under the
Attachment object storage.
For any named properties on the Attachment object, the corresponding mapping information
MUST be present in the named property mapping storage.

2.2.2.1

Embedded Message Object Storage

The .msg File Format defines separate storage semantics for Embedded Message objects. First,
as for any other Attachment object, an Attachment object storage is created for them. Any
properties on the Attachment object are stored under the Attachment object storage, as would be
done for a regular Attachment object.
Then within that Attachment object storage, a substorage with the name "__substg1.0_3701000D"
MUST be created. All properties of the Embedded Message object are contained inside this storage
and follow the regular property storage semantics.
If there are multiple levels of Attachment objects; for example, if the Embedded Message object
further has Attachment objects, they are represented by substorages contained in the Embedded
Message object storage and follow the regular storage semantics for Attachment objects. For each
Recipient object of the Embedded Message object, there is a Recipient object storage contained in
the Embedded Message object storage.
However, named property mapping information for any named properties on the Embedded
Message object MUST be stored in the named property mapping storage under the top level, and
the Embedded Message object MUST NOT contain a named property mapping storage.

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The Embedded Message object can have different Unicode state than the Message object containing
it, and so its Unicode state SHOULD be checked as specified in section 2.1.3.
It is important to understand the difference between the properties on the Attachment object and
the properties on the Embedded Message object that the Attachment object represents. An example
of a property on the Attachment object would be PidTagDisplayName ([MS-OXPROPS] section
2.746), which is a property that all Attachment objects have irrespective of whether they represent
Embedded Message objects or regular Attachment objects. Such properties are stored in the
Attachment object storage. An example of a property on an Embedded Message object is
PidTagSubject ([MS-OXPROPS] section 2.1104), and it is contained in the Embedded Message
object storage.

2.2.2.2

Custom Attachment Storage

The .msg File Format defines separate storage semantics for attachments that represent data from
an arbitrary client application. These are attachments that have the PidTagAttachMethod property
([MS-OXCMSG] section 2.2.2.9) set to afStorage (0x00000006).
First, as for any other Attachment object, an Attachment object storage is created for them. Any
properties on the Attachment object are stored under the Attachment object storage, as would be
done for a regular Attachment object.
Then, within that Attachment object storage, a substorage with the name "__substg1.0_3701000D"
is created. At this point, the application that owns the data is allowed to define the structure of the
substorage. Thus, the streams and storages contained in the custom attachment storage are defined
by the application that owns the data. For an example, see section 3.3.

2.2.3

Named Property Mapping Storage

Named properties are specified using their property names.
The mapping between a named property's property name and its property ID and vice versa is
provided by the data inside the various streams contained in the named property mapping storage.
The streams and the role each one plays are specified in the following subsections.
This storage is the one and only place where such mappings are stored for the Message object and
all its subobjects. The storage MUST be named "__nameid_version1.0".

2.2.3.1

Property ID to Property Name Mapping

The streams specified in the following sections MUST be present inside the named property mapping
storage.

2.2.3.1.1

GUID Stream

The GUID stream MUST be named "__substg1.0_00020102". It MUST store the property set GUID
part of the property name of all named properties in the Message object or any of its subobjects,
except for those named properties that have PS_MAPI or PS_PUBLIC_STRINGS, as described in
[MS-OXPROPS] section 1.3.2, as their property set GUID.
The GUIDs are stored in the stream consecutively like an array. If there are multiple named
properties that have the same property set GUID, then the GUID is stored only once and all the
named properties will refer to it by its index.

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2.2.3.1.2

Entry Stream

The entry stream MUST be named "__substg1.0_00030102" and consist of 8-byte entries, one for
each named property being stored. The properties are assigned unique numeric IDs (distinct from
any property ID assignment) starting from a base of 0x8000. The IDs MUST be numbered
consecutively, like an array. In this stream, there MUST be exactly one entry for each named
property of the Message object or any of its subobjects. The index of the entry for a particular ID is
calculated by subtracting 0x8000 from it. For example, if the ID is 0x8005, the index for the
corresponding 8-byte entry would be 0x8005 – 0x8000 = 5. The index can then be multiplied by 8
to get the actual byte offset into the stream from where to start reading the corresponding entry.
Each of the 8-byte entries has the following format:

0

1

2

3

4

5

6

7

8

1
0

9

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

1

Name Identifier/String Offset
Index and Kind Information

Name Identifier/String Offset (4 bytes): If this property is a numerical named property
(as specified by the Property Kind subfield of the Index and Kind Information field), this
value is the LID part of the PropertyName structure, as specified in [MS-OXCDATA] section
2.6.1. If this property is a string named property, this value is the offset in bytes into the
strings stream where the value of the Name field of the PropertyName structure is located.
Index and Kind Information (4 bytes): This value MUST have the structure specified in
section 2.2.3.1.2.1.

2.2.3.1.2.1

Index and Kind Information

The following structure specifies the stream indexes and whether the property is a numerical named
property or a string named property.

0

1 2

3 4

5

6

7

8

9

1
0

1

Property Index

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

GUID Index

1
Proper
ty
Kind

Property Index (2 bytes): Sequentially increasing, zero-based index. This MUST be 0 for the
first named property, 1 for the second, and so on.
GUID Index (15 bits): Index into the GUID stream. The possible values are shown in the
following table.
Value

GUID to use

1

Always use the PS_MAPI property set, as specified in [MS-OXPROPS] section 1.3.2. No
GUID is stored in the GUID stream.

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Value

GUID to use

2

Always use the PS_PUBLIC_STRINGS property set, as specified in [MS-OXPROPS] section
1.3.2. No GUID is stored in the GUID stream.

>= 3

Use Value minus 3 as the index of the GUID into the GUID stream. For example, if the
GUID index is 5, the third GUID (5 minus 3, resulting in a zero-based index of 2) is used
as the GUID for the name property being derived.

Property Kind (1 bit): Bit indicating the type of the property; zero (0) if numerical named
property and 1 if string named property.

2.2.3.1.3

String Stream

The string stream MUST be named "__substg1.0_00040102". It MUST consist of one entry for each
string named property, and all entries MUST be arranged consecutively, like in an array.
As specified in section 2.2.3.1.2, the offset, in bytes, to use for a particular property is stored in the
corresponding entry in the entry stream. That is a byte offset into the string stream from where the
entry for the property can be read. The strings MUST NOT be null-terminated. Implementers can
add a terminating null character to the string after they read it from the stream, if one is required
by the implementer's programming language.
Each entry MUST have the following format.

0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

1

Name Length
Name (variable)
...

Name Length (4 bytes): The length of the following Name field in bytes.
Name (variable): A Unicode string that is the name of the property. A new entry MUST always
start on a 4 byte boundary; therefore, if the size of the Name field is not an exact multiple of
4, and another Name field entry occurs after it, null characters MUST be appended to the
stream after it until the 4-byte boundary is reached. The Name Length field for the next
entry will then start at the beginning of the next 4-byte boundary.

2.2.3.2

Property Name to Property ID Mapping Streams

Besides the three streams that provide a map of property IDs to property names, there MUST be
streams in the named property mapping storage that provide a map of property names to property
IDs. Each named property MUST have an entry in one of those streams, although one stream can
have entries for multiple named properties. The following sections specify the steps for creating the
property name to property ID mapping stream.

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2.2.3.2.1

Determining GUID Index

The first step in creating the property name to property ID mapping stream is to determine the
GUID index. The GUID index for a named property is computed from the position at which its GUID
is stored in the GUID stream, except if the GUID is that of the PS_MAPI or PS_PUBLIC_STRINGS
property set, as specified in [MS-OXPROPS] section 1.3.2. The following table specifies how the
GUID index is computed.

Property set

GUID
index

PS_MAPI

1

PS_PUBLIC_STRINGS

2

Other property sets: Search for the GUID in the GUID stream. If the GUID is the first one in
the GUID stream, the GUID index is 3; if it is the second GUID in the GUID stream, the GUID
index is 4, and so on.

Index +
3

Index is the zero-based position of the GUID in the GUID stream.

2.2.3.2.2

Generating Stream ID

The second step in creating the property name to property ID mapping stream is to generate the
stream ID. The stream ID is a number used to create the name of the stream for the named
property.
The stream ID for a particular named property is calculated differently depending on whether the
named property is a numerical named property or a string named property.

2.2.3.2.2.1

Stream ID Equation

For numerical named properties, the following equation is used:
Stream ID = 0x1000 + ((ID XOR (GUID index << 1))) MOD 0x1F

For string named properties, the following equation is used:
Stream ID = 0x1000 + ((ID XOR (GUID index << 1 |1))) MOD 0x1F

0x1F is the maximum number of property name to property ID mapping streams that the .msg File
Format allows in the named property mapping storage.
For numerical named properties, ID, in the equation, is the name identifier.
For string named properties, ID is generated by computing the CRC-32 (cyclic redundancy check
(CRC)) for the property's Unicode name identifier.<1> For more information on the CRC-32
algorithm, see [X25].

2.2.3.2.3

Generating Stream Name

The third step in creating the property name to property ID mapping stream is to use the stream ID
to generate a hexadecimal identifier. The hexadecimal identifier is a ULONG ([MS-DTYP]) and is

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generated in this case by setting the first 16 bits to be the stream ID and the last 16 bits to be
0x0102. The computation of the hexadecimal identifier is as follows:
Hexadecimal Identifier = (stream ID << 16) | 0x00000102

The stream name is then generated by prefixing the hexadecimal identifier with the following string:
"__substg1.0_". For example, if the stream ID is 0x100A, the hexadecimal identifier is 0x100A0102
and the stream name is "__substg1.0_100A0102".
Multiple named properties can be mapped to the same stream if the same stream ID is generated
by the stream ID equation.

2.2.3.2.4

Obtaining Stream Data

Each of these streams MUST be an array of 8-byte entries. The number of entries in one stream
depends on the number of properties that were mapped into it by the stream ID equation. Each 8byte entry MUST have the following structure.

0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

1

Name Identifier/CRC-32 Checksum
Index and Kind Information

Name Identifier/CRC-32 Checksum (4 bytes): If this property is a numerical named
property, this value is the name identifier obtained from the stream. By comparing this value
with the name identifier obtained from the property name, the correct 8-byte entry can be
identified. If this property is a string named property, this value is the CRC-32 checksum
obtained from the stream. By comparing this value with the CRC-32 computation of the
Unicode string name, the correct 8-byte entry can be identified.
Index and Kind Information (4 bytes): This field contains an Index and Kind Information
structure, as specified in section 2.2.3.1.2.1.
Once the correct entry is identified, the property ID of the named property is simply the sum of
0x8000 and the value of the Property Index field of the Index & Kind Information structure. An
example illustrating this mapping is provided in section 3.2.2.

2.3

Top Level Structure

The top level of the file represents the entire Message object. 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 the properties that are set on it.
The .msg File Format specifies the following top level structure. Under the top level are the
following:
Exactly one Recipient object storage for each Recipient object of the Message object.
Exactly one Attachment object storage for each Attachment object of the Message object.
Exactly one named property mapping storage.

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Exactly one property stream, and it MUST contain entries for all properties of the Message object.
Exactly one stream for each variable length property of the Message object. That stream MUST
contain the value of that variable length property.
Exactly one stream for each fixed length multiple-valued property of the Message object. That
stream MUST contain all the values of that fixed length multiple-valued property.
For each variable length multiple-valued property of the Message object, if there are N values,
there MUST be N + 1 streams.

2.4

Property Stream

The property stream MUST have the name "__properties_version1.0" and MUST consist of a header
followed by an array of 16-byte entries. With the exception of Named Property Mapping storage,
which is specified in section 2.2.3, every storage type specified by the .msg File Format MUST have
a property stream in it.
Every property of an object MUST have an entry in the property stream for that object. Fixed length
properties also have their values stored as a part of the entry, whereas the values of variable length
properties and multiple-valued properties are stored in separate streams.

2.4.1

Header

The header of the property stream differs depending on which storage this property stream belongs
to.

2.4.1.1

Top Level

The header for the property stream contained inside the top level of the .msg file, which represents
the Message object itself, has the following structure.

0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

1

Reserved
...
Next Recipient ID
Next Attachment ID
Recipient Count
Attachment Count
Reserved
...

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Reserved (8 bytes): This field MUST be set to zero when writing a .msg file and MUST be
ignored when reading a .msg file.
Next Recipient ID (4 bytes): The ID to use for naming the next Recipient object storage if one
is created inside the .msg file. The naming convention to be used is specified in section 2.2.1.
If no Recipient object storages are contained in the .msg file, this field MUST be set to 0.
Next Attachment ID (4 bytes): The ID to use for naming the next Attachment object storage
if one is created inside the .msg file. The naming convention to be used is specified in section
2.2.2. If no Attachment object storages are contained in the .msg file, this field MUST be set
to 0.
Recipient Count (4 bytes): The number of Recipient objects.
Attachment Count (4 bytes): The number of Attachment objects.
Reserved (8 bytes): This field MUST be set to 0 when writing a .msg file and MUST be ignored
when reading a .msg file.

2.4.1.2

Embedded Message object Storage

The header for the property stream contained inside any Embedded Message object storage has the
following structure.

0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

1

Reserved
...
Next Recipient ID
Next Attachment ID
Recipient Count
Attachment Count

Reserved (8 bytes): This field MUST be set to zero when writing a .msg file and MUST be
ignored when reading a .msg file.
Next Recipient ID (4 bytes): The ID to use for naming the next Recipient object storage if one
is created inside the .msg file. The naming convention to be used is specified in section 2.2.1.
Next Attachment ID (4 bytes): The ID to use for naming the next Attachment object storage
if one is created inside the .msg file. The naming convention to be used is specified in section
2.2.2.
Recipient Count (4 bytes): The number of Recipient objects.
Attachment Count (4 bytes): The number of Attachment objects.

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2.4.1.3

Attachment Object Storage or Recipient Object Storage

The header for the property stream contained inside an Attachment object storage or a Recipient
object storage has the following structure.

0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

1

Reserved
...

Reserved (8 bytes): This field MUST be set to zero when writing a .msg file and MUST be
ignored when reading a .msg file.

2.4.2

Data

The data inside the property stream MUST be an array of 16-byte entries. The number of properties,
each represented by one entry, can be determined by first measuring the size of the property
stream, then subtracting the size of the header from it, and then dividing the result by the size of
one entry.
The structure of each entry, representing one property, depends on whether the property is a fixed
length property or not.

2.4.2.1

Fixed Length Property Entry

The entry for a fixed length property has the following structure.

0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

1

Property Tag
Flags
Value
...

Property Tag (4 bytes): The property tag of the property.
Flags (4 bytes): Flags giving context to the property. Possible values for this field are given in
the following table. Any bitwise combination of the flags is valid.
Flag name

Value

Description

PROPATTR_MANDATORY

0x00000001

If this flag is set for a property, that property MUST
NOT be deleted from the .msg file (irrespective of
which storage it is contained in) and
implementations MUST return an error if any attempt

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Flag name

Value

Description
is made to do so. This flag is set in circumstances
where the implementation depends on that property
always being present in the .msg file once it is
written there.

PROPATTR_READABLE

0x00000002

If this flag is not set on a property, that property
MUST NOT be read from the .msg file and
implementations MUST return an error if any attempt
is made to read it. This flag is set on all properties
unless there is an implementation-specific reason to
prevent a property from being read from the .msg
file.

PROPATTR_WRITABLE

0x00000004

If this flag is not set on a property, that property
MUST NOT be modified or deleted and
implementations MUST return an error if any attempt
is made to do so. This flag is set in circumstances
where the implementation depends on the properties
being writable.

Value (8 bytes): This field contains a Fixed Length Property Value structure, as specified in
section 2.4.2.1.1.

2.4.2.1.1

Fixed Length Property Value

The following structure contains the value of the property.

0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

3
0

9

1

Data (variable)
Reserved (variable)

Data (variable): The value of the property. The size of this field depends upon the property
type, which is specified in the Property Tag field, as specified in section 2.4.2.1. The size
required for each property type is specified in [MS-OXCDATA] section 2.11.1.
Reserved (variable): This field MUST be ignored when reading a .msg file. The size of the
Reserved field is the difference between 8 bytes and the size of the Data field; if the size of
the Reserved field is greater than 0, this field MUST be set to 0 when writing a .msg file.

2.4.2.2

Variable Length Property or Multiple-Valued Property Entry

The entry for a variable length property has the following structure.

0

1

2

3

4

5

6

7

8

9

1
0

1

2

3

4

5

6

7

8

9

2
0

1

2

3

4

5

6

7

8

9

3
0

1

Property Tag

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Flags
Size
Reserved

Property Tag (4 bytes): Same as the description in section 2.4.2.1.
Flags (4 bytes): Same as the description in section 2.4.2.1.
Size (4 bytes): This value is interpreted based on the property type, which is specified in the
Property Tag field. If the message contains an embedded message attachment or a storage
attachment, this field MUST be set to 0xFFFFFFFF. Otherwise, the following table shows how
this field is interpreted for each property type. The property types are specified in [MSOXCDATA] section 2.11.1.
Property type

Meaning of Size value

Variable length
property, except for
PtypString or
PtypString8

Size MUST be equal to the size of the stream where the value of the
property represented by this entry is stored.

PtypString

Size MUST be equal to 2 plus the size of the stream where the value of
the property represented by this entry is stored. The string being stored
MUST<2> have at least one character. When parsing property streams,
clients MUST issue a MAPI_E_BAD_VALUE error for any zero-length
property streams of PtypString.

PtypString8

Size MUST be equal to 1 plus the size of the stream where the value of
the property represented by this entry is stored. The string being stored
MUST have at least one character. When parsing property streams, clients
MUST issue a MAPI_E_BAD_VALUE error for any zero-length property
streams of PtypString8.

Multiple-valued fixed
length property

Size MUST be equal to the size of the stream where all values of the
property represented by this entry are stored.

Multiple-valued
variable length
property

Size MUST be equal to the size of the length stream where the lengths of
the value streams for the property represented by this entry are stored.

Reserved (4 bytes): This field MUST be ignored when reading a .msg file. When writing a .msg
file, this field MUST be set to 0x01 if the message contains an embedded message attachment
and to 0x04 if the message contains a storage attachment. The following table shows the
required value for this field based on the value of the PidTagAttachMethod property ([MSOXCMSG] section 2.2.2.9).
PidTagAttachMethod property value

Required Reserved field value

ATTACH_EMBEDDED_MSG (0x05)

0x01

ATTACH_OLE (0x06)

0x04

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3

Structure Examples

3.1

From Message Object to .msg File

The structure of a Message object in the .msg File Format that has two Attachment objects and two
Recipient objects is represented in figure 1. In the figures, the folder icons represent storages, and
the text page icons represent streams. Note that the streams present depend on the properties that
are set on the Message object.

Figure 1: A sample message in the .msg File Format
A few things to note:
"__nameid_version1.0" is the named property mapping storage that contains all named property
mappings for the Message object and its subobjects.
"__properties_version1.0" is the property stream.
"__recip_version1.0_#00000000" and "__recip_version1.0_#00000001" are Recipient object
storages, each representing one Recipient object of the Message object.
"__attach_version1.0_#00000000" and "__attach_version1.0_#00000001" are Attachment
object storages, each representing one Attachment object in the Message object.

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An expanded view of the "__nameid_version1.0" named property mapping storage is shown in the
following figure.

Figure 2: Expanded view of the named property mapping storage
In the preceding figure, the "__nameid_version1.0" named property mapping storage contains the
three streams used to provide a mapping from property ID to property name
("__substg1.0_00020102", "__substg1.0_00030102", and "__substg1.0_00040102") and various
other streams that provide a mapping from property names to property IDs.
An expanded view of the "__recip_version1.0_#00000000" and "__recip_version1.0_#00000001"
Recipient object storages and the "__attach_version1.0_#00000000" and
"__attach_version1.0_#00000001" Attachment object storages is shown in the following figure.

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Figure 3: Expanded view of Attachment object storages and Recipient object storages

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In the preceding figure, each of the Attachment object storages and Recipient object storages
contain the property stream (1) and a stream for each variable length property. One of the
Attachment objects is itself a Message object, and it has a substorage called
"__substg1.0_3701000D" where properties pertaining to that Message object are stored. The
Embedded Message object storage contains a Recipient object storage and six Attachment object
storages.

3.2

Named Property Mapping

In this example that illustrates how named property mapping works, it is assumed that the named
property mapping storage has been populated with the data required to achieve named property
mapping, as specified by the .msg File Format.

3.2.1

Property ID to Property Name

For both numerical named properties and string named properties, the first step in mapping a
property name to a property ID is to fetch the entry from the entry stream. Once the kind of the
named property has been determined, the logic for fetching the name identifier is different.

3.2.1.1

Fetching the Name Identifier

In this example, property ID 0x8005 has to be mapped to its property name. First, the entry index
into the entry stream is determined:
Property ID – 0x8000
=0x8005 – 0x8000
=0x0005
Then, the offset for the corresponding 8-byte entry is determined:
Entry index * size of entry
= 0x05 * 0x08
= 0x28
The offset is then used to fetch the entry from the entry stream ("__substg1.0_00030102"), which
is contained inside the named property mapping storage ("__nameid_version1.0"). In this case,
bytes 40 – 47 are fetched from the stream. Then, the structure specified in the entry stream section
is applied to those bytes, taking into consideration that the data is stored in little-endian format.

3.2.1.1.1

Numerical Named Property

The following 8 bytes represent an entry from the entry stream (in hexadecimal notation):
1C 81 00 00 08 00 05 00

The structure specified in the entry stream section is applied to these bytes to obtain the following
values:
Name identifier = 0x811C
Property index = 0x05

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GUID index = 0x04
Property Kind= 0
From these values, it is determined that this is a numerical named property that has the name
identifier 0x811C.

3.2.1.1.2

String Named Property

The following 8 bytes represent an entry from the entry stream (in hexadecimal notation):
10 00 00 00 07 00 05 00

The structure specified in the entry stream section is applied to these bytes to obtain the following
values:
String offset = 0x10
Property index = 0x05
GUID index = 0x03
Property Kind = 1
From these values it is determined that this is a string named property with a string offset of 0x10.
The string offset is then used to fetch the entry from the string stream ("__substg1.0_00040102"),
which is contained inside the named property mapping storage ("__nameid_version1.0"). The
structure in the table specified in the string stream section is applied to those bytes, taking into
consideration that the data is stored in little-endian format.
If the string stream is as follows:
09 92 7D 46 35 2E 7D 1A 41 11 92 72 01 F2 30 12 00 00 00 1C 00 5A 00 5C 00 91 00 48 00 45 00
44 00 41 00 45 00 52 00 20 00 53 00 49 00 5A 00 44 8A 6F BB 4D 12 52 E4 11 09 91

The 4 bytes at offset 0x10 constitute the ULONG ([MS-DTYP]) 0x0000001C. The string name starts
at 0x10 + 0x04 = 0x14 and extends till 0x14 + 0x1C = 0x2F. Hence, it will be the following:
00 5A 00 5C 00 91 00 48 00 45 00 44 00 41 00 45 00 52 00 20 00 53 00 49 00 5A 00 44

3.2.1.2

Fetching the GUID

The only missing piece of information at this point is the GUID. It is fetched by first calculating the
GUID Entry Index:
GUID index – 0x03
= 0x04 – 0x03
= 0x01
Then the offset into the GUID stream is determined:

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GUID Entry Index * size of GUID
=0x01 * 0x10
= 0x10
The offset is then used to fetch the GUID from the GUID stream ("__substg1.0_00020102"), which
is contained inside the named property mapping storage ("__nameid_version1.0"). In this case,
bytes 16 – 31 will be fetched from the stream.
In this example, the 16 bytes fetched are as follows (in hexadecimal notation):
03 20 06 00 00 00 00 00 C0 00 00 00 00 00 00 46

Considering that the bytes are in little-endian format, the GUID is as follows:
{0x00062003, 0x0000, 0x0000, {0xC0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x46}}
Thus all the fields needed to specify the property name, given a property ID, can be obtained from
the data stored in the entry stream, the string stream, and the GUID stream.

3.2.2

Property Name to Property ID

If a property name is specified, the data inside the named property mapping storage is used to
determine the property ID of the property. The method differs slightly for string named properties
and numerical named properties.
If the property name specified is the following:
GUID = {0x00062003, 0x0000, 0x0000, {0xC0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x46}}
Name identifier = 0x811C
Kind = 0
First the GUID is examined to compute the GUID index, as described in section 2.2.3.2.1.
In this example, the GUID was found in the second position in the GUID stream, so its GUID index
will be 0x04.
Then, the stream ID is calculated using the stream ID equation for numerical named properties:
0x1000 + (name identifier XOR (GUID index << 1)) MOD 0x1F
= 0x1000 + (0x811C XOR (0x04 << 1)) MOD 0x1F
= 0x1000 + (0x811C XOR 0x08) MOD 0x1F
= 0x1000 + 0x8114 MOD 0x1F
= 0x1000 + 0x1D
= 0x101D
Then, the hexadecimal identifier is generated as follows:
stream ID << 16 | 0x00000102

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= 0x101D << 16 | 0x00000102
= 0x101D0102
The stream name is generated by concatenating "__substg1.0_" and the hexadecimal identifier.
Therefore, the stream name is "__substg1.0_101D0102".
The data inside the stream is an array of 8-byte entries, each with the structure described in section
2.2.3.2.4. One of those entries maps to the named property in question and can be found by
comparing the name identifier of the named property with that fetched from the stream. In this
example, the stream "__substg1.0_101D0102" has the following contents:
1C 81 00 00 08 00 05 00 15 85 00 00 06 00 40 00 34 85 00 00 06 00 4A 00 A8 85 00 00 06 00 70
00

The structure described in section 2.2.3.2.4 is applied to these bytes to obtain the following entries.
Serial #

Name identifier

Property index

GUID index

Property Kind

1

0x811C

0x05

0x04

0

2

0x8515

0x40

0x03

0

3

0x8534

0x4A

0x03

0

4

0x85A8

0x70

0x03

0

The entry corresponding to the named property in question is number 1 because the name identifier
from the stream is equal to the property's name identifier.
The property ID is then computed as follows:
0x8000 + property index
= 0x8000 + 0x05
= 0x8005

3.3

Custom Attachment Storage

The storage format of attachments that represent data from an arbitrary client application is
controlled by the application itself. For example, a media application can write a completely different
set of streams under the substorage than an image manipulation application.
The following figure shows the structure of the substorage for two different types of applications,
demonstrating that the structure is essentially controlled by the owning application.

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Figure 4: Expanded view of the substorage for two different types of applications

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4

Security

4.1

Security Considerations for Implementers

The .msg File Format provides some 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.
However, there are certain inherent security concerns with .msg files:
Possible modification of properties, especially security-related flags.
The .msg File Format does not provide for any encryption.

4.2

Index of Security Parameters

None.

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5

Appendix A: Product Behavior
The information in this specification is applicable to the following Microsoft products or supplemental
software. References to product versions include released service packs:
Microsoft Exchange Server 2003
Microsoft Exchange Server 2007
Microsoft Exchange Server 2010
Microsoft Exchange Server 2013
Microsoft Office Outlook 2003
Microsoft Office Outlook 2007
Microsoft Outlook 2010
Microsoft Outlook 2013
Exceptions, if any, are noted below. If a service pack or Quick Fix Engineering (QFE) number
appears with the product version, behavior changed in that service pack or QFE. The new behavior
also applies to subsequent service packs of the product unless otherwise specified. If a product
edition appears with the product version, behavior is different in that product edition.
Unless otherwise specified, any statement of optional behavior in this specification that is prescribed
using the terms SHOULD or SHOULD NOT implies product behavior in accordance with the SHOULD
or SHOULD NOT prescription. Unless otherwise specified, the term MAY implies that the product
does not follow the prescription.
<1> Section 2.2.3.2.2.1: If the string named property belongs to the PS_INTERNET_HEADERS
property set ([MS-OXPROPS] section 1.3.2), then the Office Outlook 2003, Office Outlook 2007, and
Outlook 2010 implementations of the .msg File Format will convert the Unicode property name to
lower case before computing the equivalent CRC-32 for it.
<2> Section 2.4.2.2: Office Outlook 2003, Office Outlook 2007 and Outlook 2010 will not open a
.msg file with zero-length property streams of type PtypString or PtypString8.

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6

Change Tracking
No table of changes is available. The document is either new or has had no changes since its last
release.

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7

Index
.msg file properties
PidTagStoreSupportMask 9
.msg file properties other properties 9

multiple-valued properties 10

A

Named Property Mapping example 28
Named property mapping example - property ID to
property name 28
Named property mapping example - property name
to property ID 30
Named property mapping storage 15
Normative references 6

Applicability 8
Attachment object storage 14

C
Change tracking 35
Compound files 7
Custom Attachment Storage example 31

D
Data - property stream 22
Details
fixed length properties 9
multiple-valued properties 10
other properties 9
PidTagStoreSupportMask 9
Properties structure 9
variable length properties 10

N

O
Overview
compound files 7
properties 7
storages 7
top level structure 7
Overview (synopsis) 6

P

Fields - vendor-extensible 8
fixed length properties 9
From Message Object to .msg File example 25

Parameters - security index 33
Product behavior 34
Properties 7
fixed length properties 9
multiple-valued properties 10
other properties 9
PidTagStoreSupportMask 9
variable length properties 10
Properties structure 9
Property ID to property name example 28
Property name to property ID example 30
Property stream
data 22
header 20
Property stream structures 20

G

R

Glossary 5

Header - property stream 20

Recipient object storage 13
References 6
informative 6
normative 6
Relationship to protocols and other structures 7

I

S

Implementer - security considerations 33
Index of security parameters 33
Informative references 6
Introduction 5

Security
implementer considerations 33
parameter index 33
Storages
Attachment object storage 14
named property mapping storage 15
overview 7
Recipient object storage 13
Structures
properties 9

E
Examples
Custom Attachment Storage 31
From Message Object to .msg File 25
Named Property Mapping 28

F

H

L
Localization 8

M

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property stream 20
top level structure 19

T
Top level structure 19
Top level structure - overview 7
Tracking changes 35

V
variable length properties 10
Vendor-extensible fields 8
Versioning 8

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