multipart/related
The MIME type (RFC 2387) for parts meant to be used together, such as an HTML body and the images it embeds through cid: references.
What it means
multipart/related bundles a root part, normally the HTML, with resources it refers to. Each image part has a Content-ID header, and the HTML points to it with a cid: URL (RFC 2392), as in <img src="cid:logo@example.com">, so the image displays without being fetched from the web. By default the first part is the root; a start parameter can name another.
A message with embedded images and attachments nests these types, for example a multipart/mixed containing a multipart/related, which in turn holds the multipart/alternative body and the images. Embedded images make the message larger; the alternative is to host images on a web server, accepting that remote images can be blocked until the reader allows them.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2387: The MIME Multipart/Related Content-typerfc-editor.org
- RFC 2392: Content-ID and Message-ID Uniform Resource Locatorsrfc-editor.org
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