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7. Content-ID Header Field

In certain cases, such as when using the "multipart/related" content type, it is useful to be able to refer to the specific body parts of a MIME message. The Content-ID header field provides a mechanism for uniquely identifying a MIME entity.

Syntax​

id := "Content-ID" ":" msg-id

The syntax of msg-id is defined in RFC 822. As with all other entity header fields, this is an optional field.

Examples​

Content-ID: ``<[email protected]>``

Uses​

The Content-ID field can be used for:

  • Referencing embedded resources in multipart/related messages
  • Referencing inline images in HTML mail
  • Uniquely identifying specific parts of a message

Key Points:

  • Provides unique identifier
  • Uses RFC 822 msg-id syntax
  • Optional field
  • Commonly used in multipart/related
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