2.1.3.3.5 HTML Text Message Body from RTF Without Inline Attachments
When the best body format type is RTF and no inline (OLE) Attachment objects exist, MIME writers SHOULD generate a multipart/alternative MIME entity with the following two child entities:
The first child entity has "text/plain" for the value of its Content-Type header. Its body SHOULD be plain text generated from the value of the PidTagRtfCompressed property ([MS-OXCMSG] section 2.2.1.58.4). MIME writers that rely on the store for text conversion MAY instead copy the body from the value of the PidTagBody property ([MS-OXCMSG] section 2.2.1.58.1), assuming that both properties contain substantially similar text. The format of the RTF data is specified in [MSFT-RTF].
The second child entity has "text/HTML" for the value of its Content-Type header. Its body is HTML text. The HTML text SHOULD be generated from the value of the PidTagRtfCompressed property ([MS-OXPROPS] section 2.943). MIME writers that rely on the store for text conversion MAY instead copy the body from the value of the PidTagHtml property ([MS-OXCMSG] section 2.2.1.58.9), assuming that both properties contain substantially similar text.
MIME content-types of type "text/" and the charset parameter SHOULD be processed as specified in section 2.1.3.3.4.