2.1.3.5.2 TNEF Encapsulation Versus Outer Wrapper Attributes

It is possible to encapsulate properties in the TNEF stream that represents data that is already conveyed as part of an outer wrapper such as MIME. Because in many cases the outer wrapper properties are subject to substantial validation during transport (reverse DNs lookups and other validation), they are considered more trustworthy than information inside an attached file that otherwise could overwrite correct information with incorrect information.

As a general rule, the TNEF Writer does not duplicate any data that is already being reliably transported outside the stream and the TNEF Reader does not override data from outside the stream with data from inside the stream. Individual implementations determine how this rule is applied. For more details, see [MS-OXCMAIL]. Some highlights of this behavior are specified in the following sections.