188.8.131.52 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.
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 2.676), 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 2.1033), and it is contained in the Embedded Message object storage.