The Content-ID header associates a unique ID with a MIME part. In Figure 32, the Content-ID header contains the value <part00909@roguewave.example>. A Content-ID header is designed to allow external references to a given MIME part. Therefore, the Content-ID provided must be world-unique.

The value of a Content-ID header has the format <xxxx@yyyy>. A MIME application typically creates a world-unique value by setting the right side of the value to indicate the system where the MIME part originated. The left side of the value generally contains a serial number, an indication of the date and time the message was generated, or both. Most applications use a combination of letters, numbers, and common punctuation (typically, period and hyphen) for the values of xxxx and yyyy, even though the specification technically allows a slightly larger character set.

Most MIME messages contain a single Content-ID header, at the top level of the message. If the Content-Type of the part is m-ultipart/alternative, the MIME specification recommends that each contained part have a unique Content-ID unless each part contains exactly the same information (see RFC 2046 for complete details).

RFC 2045 defines the Content-ID header in section 7 (p. 26).