Header fields provide information about the structure and encoding of a message. They consist of name:value pairs and appear at the top of the message. A MIME-compliant message must contain the “MIME-Version” header field.

Besides the MIME-Version header field, most messages have additional fields that supply information to the agent, transport, or application that will convey or consume the message. For example, when a MIME message carries anything other than plain, US-ASCII text, it must include the “Content-Type” header field. Messages that are routed over SMTP will also have the “Date,” “To,” and “From” header fields.

A message may also contain custom header fields that are specific to a particular agent or application. Such application-specific header fields must be prefixed with the characters “X-” to distinguish them from the standard header fields defined by the MIME and/or transport protocols.

This chapter does not attempt to describe the purpose or use of individual header fields. However, to use MIME effectively, you will need to understand which header fields your solution requires and know how to set them or interpret them correctly. For information about header fields, see the following references.