Description of multipart/mixed Internet message format - Exchange

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A multipart/mixed MIME message is composed of a mix of different data types. Each body part is delineated by a boundary. The boundary parameter is a text string used to delineate one part of the message body from another. All boundaries start with two hyphens (--). The final boundary also concludes with two hyphens (--). The boundary can be made up of any ASCII character except for a space, a control character, or special characters.

When Exchange Server sends MIME messages, the content-type depends on whether there are attachments to the message, and on the formatting of the message text. If there are attachments, the content-type is multipart/mixed. In this case, the message text and each attachment become a separate part of the message content, each with its own content-type. If there are no attachments, the content-type of the message is Text/Plain, and the message body is made up of only one part.

A multipart/mixed MIME message header of a message sent with a Microsoft Word attachment may appear similar to the following:

Content-type: multipart/mixed;
boundary="Boundary_any ascii character except some of the following special characters:
<BR/> ( )< > @ , ; : \ / [ ] ? = "
--Boundary_any ASCII character, except some special characters below:
content-Type: text/plain;----
charset=iso-8859-1
Content-transfer-encoding: 7BIT
--Boundary_ASCII characters  
Content-type: application/msword;
name="message.doc"
Content-Transfer-Encoding: base64
Messages that are composed by MIME clients as a multipart/mixed MIME message without a name parameter for the content-type or an optional filename parameter for the content-disposition header may not be rendered correctly when received by Exchange.