Content-ID

This header field is typically added to message parts within a multipart/related message. The multpart/related usually has an HTML document as its main part, and then embedded images, style sheets, and other presentation elements in subsequent MIME message parts. Rather than using external URLs, the HTML can use "CID" links to reference the other MIME message parts. For example: <img src="cid:123xyzerhgieurgdkflg@abc"> The Content-ID value is enclosed in angle brackets.

Example:

Content-ID: <123xyzerhgieurgdkflg@abc>