The basic structure of a MIME part consists of a list of headers and a body. The key feature of the MIME specification is a standardized set of headers that describe the structure and content of the part. Each header field consists of a label and a value. Some values contain one or more parameters. For example, the header:

contains the label Content-Type and the value text/plain; charset=us-ascii. The value contains the content type text/plain and a single parameter. The parameter contains the name charset and the value us-ascii. In short, the header describes the part body as plain text in the US-ASCII character set. The only role of a MIME header is to describe the content of the part or identify the MIME version used. MIME headers don’t have anything to do with the transport protocol used for the message.

The body of a MIME part is the content of the part. The content can be simple data, such as unstructured text or an encoded image. Alternatively, the part can contain other MIME parts. Each nested MIME part contains its own headers and body.