multipart/alternative
A MIME structure holding several versions of the same content, typically plain text and HTML, from which the mail program displays the best one it can.
What it means
RFC 2046 says the parts of multipart/alternative carry the same information in different formats, placed in increasing order of preference, with the preferred format last. In email that means the plain-text version first and the HTML version last. A program that can render HTML shows it; one that cannot, or a reader who prefers text, gets the text part.
Both versions should say the same thing and carry the same links, since some readers will only ever see one of them. The HTML part can itself be a multipart/related structure when the message embeds images.
Example
Content-Type: multipart/alternative; boundary="alt"
--alt
Content-Type: text/plain; charset=utf-8
(plain-text version)
--alt
Content-Type: text/html; charset=utf-8
(HTML version)
--alt--Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2046, section 5.1.4: Alternative Subtyperfc-editor.org
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