MIME
Multipurpose Internet Mail Extensions: the standards (RFC 2045 to 2049) that let email carry other character sets, HTML, images, attachments and messages made of several parts.
What it means
RFC 5322 mail is plain US-ASCII text. MIME adds header fields that describe the content: MIME-Version: 1.0, Content-Type (what the content is, and its character set), Content-Transfer-Encoding (how it was made safe for transport, such as base64 or quoted-printable) and Content-ID, with Content-Disposition (RFC 2183) for attachments. Multipart types let one message carry several parts separated by boundary lines, nested as deeply as needed.
A typical marketing message is multipart/alternative with a text and an HTML version; one with attachments wraps that in multipart/mixed; embedded images add multipart/related. Malformed MIME, such as a missing boundary or a part with the wrong encoding, makes messages render badly, and Gmail rejects malformed messages with 554 5.6.0.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2045: Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodiesrfc-editor.org
- RFC 2046: Multipurpose Internet Mail Extensions (MIME) Part Two: Media Typesrfc-editor.org
- RFC 2183: Communicating Presentation Information in Internet Messages: The Content-Disposition Header Fieldrfc-editor.org
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
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