188.8.131.52.6 HTML Text Message Body with Inline Attachments
When the best body format type is HTML and inline Attachment objects exist, MIME writers SHOULD generate a MIME entity with "multipart/related" for the value of its Content-Type header and two or more child elements, as follows:
Subsequent child entities are generated from the Message object's inline Attachment objects. A child entity MUST be generated if and only if the Attachment object is marked as specified in section 184.108.40.206.1.
MIME writers SHOULD verify that the HTML text actually contains a reference to each inline Attachment object, either by its PidTagAttachContentId property ([MS-OXCMSG] section 220.127.116.11) or its PidTagAttachContentLocation property ([MS-OXCMSG] section 18.104.22.168), as specified in 22.214.171.124.1. If the HTML text contains no such reference, the MIME writer SHOULD<68> consider this Attachment object as not inline and generate its MIME entity as a peer of the MIME entity with a MIME content-type of " multipart/related" instead of as its child.