220.127.116.11.4 HTML Text Message Body Without Inline Attachments
When the best body format type is HTML and no inline Attachment objects exist, MIME writers SHOULD generate a MIME entity that has "multipart/alternative" for the value of its Content-Typeheader (2), and that has the following two child entities:
The first child entity has "text/plain" for the value of its Content-Type header (2). Its body SHOULD be plain text generated from the value of the PidTagHtml property ([MS-OXCMSG] section 18.104.22.168.9). MIME writers that rely on the store for text conversions MAY<67> instead copy the body from the value of the PidTagBody property ([MS-OXCMSG] section 22.214.171.124.1), assuming that the value of the PidTagBody property is equal to the value of the PidTagHtml propertyhttps://msdn.microsoft.com/en-us/library/cc433490(v=exchg.80).aspx.
The second child entity has "text/HTML" for the value of its Content-Type header (2). Its body is the value of the PidTagHtml property.
The plain text and charset parameters SHOULD be processed as specified in section 126.96.36.199.3. HTML text MAY be processed in exactly the same way, or characters that do not fit the preferred character set (2) can instead be encoded within the HTML.