A MIME message consists of both data and metadata. MIME metadata consists of HTTP-style headers and MIME boundary delimiters.
Each header is a colon-separated name-value pair on a line. The ASCII sequence <CR><LF> terminates the line. A sequence of these headers, called a header block, is terminated by a blank line: <CR><LF><CR><LF>. Any headers that are in this HTTP style can appear in a MIME document. Some common MIME headers are described in MIME standard header fields.
The only header that must be present is the Content-Type header. This header specifies the type of the data in the message. If the Content-Type value starts with "multipart", the message is a multipart MIME message. For multipart messages the Content-Type header must also include a boundary attribute that gives the text that is used to delimit the message parts. Each MIME part has its own Content-Type field that specifies the type of the data in the part. This can also be multipart, which allows multipart messages to be nested. MIME parts with any other Content-Type values are handled as BLOB data.
SET OutputRoot.Properties.ContentType = 'text/plain';
S/MIME is a standard for sending secure email messages. S/MIME has an outer level Content-Type of multipart/signed with parameters protocol and micalg that define the algorithms that are used to encrypt the message. One or more MIME parts can have encoded content. These parts have Content-Type values such as application/pkcs7-signature and a Content-Transfer-Encoding of base64. The MIME domain does not attempt to interpret or verify whether the message is signed.