TNEF Correlation Key

When creating a new TNEF message, MIME writers choose a unique key relating the TNEF body part to its parent message. (MIME writers SHOULD use the value of the PidTagInternetMessageId property ([MS-OXOMSG] section for this purpose.) The chosen value SHOULD<56> be written in the following two places:

This pair of values SHOULD be used by MIME writers to validate that the top-level message and its TNEF body part do, in fact, belong to each other and are not (for example) the result of a non-TNEF-aware Mail User Agent (MUA) forwarding a message with an attached TNEF body part and retaining the attachment.