Programming Internet Email

MIME Header Fields

MIME headers come in two flavors: MIME message headers and MIME part headers. MIME message headers are just additional RFC 822-style message headers. They denote that a message is MIME compliant and inform a receiving MUA of the structure and encoding of the message. MIME part headers reside in a message body and describe the contents of each part of a multipart message.

If a MIME header is part of a message header block, it applies to the entire message. If it appears at the beginning of a message part, it applies only to that part.

MIME message headers are:

• MIME-Version

• Content-Type

• Content-Transfer-Encoding

• Content-ID

• Content-Description

• Content-Disposition (experimental)

MIME part headers can be any valid MIME header except the MIME version header. In other words, MIME part headers must begin with “Content”.

If a later MIME version is defined, its headers will start with “Content-”. As with RFC 822 messages, any user-defined headers that are used should start with “X-” to avoid any potential conflicts with emerging standards.

MIME headers, whether message headers or part headers, may occur in any order.

MIME messages have, at least, the minimum simple text headers (described in Chapter 2) and additional MIME headers. These are called MIME message headers, to distinguish them from the headers used at the beginning of each body part, the so-called MIME part headers. The MIME message headers give the MIME version, the type of message structure used, and the string that separates ...