2.4 Property Stream
The property stream (1) 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 (1) in it.
Every property of an object MUST have an entry in the property stream (1) 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 (1).