http://www.rfc-editor.org/rfc/rfc2111.txt
RFC 2111: Content-ID and Message-ID Uniform Resource Locators, E. Levinson1997-03-01T23:00:00-00:00The Uniform Resource Locator (URL) schemes, "cid:" and "mid:" allow references to messages and the body parts of messages. For example, within a single multipart message, one HTML body part might include embedded references to other parts of the same message. [STANDARDS-TRACK]