multipart/mixed
The MIME type for a message whose parts are independent and kept in a set order, most often a message body followed by attachments.
What it means
RFC 2046 defines multipart/mixed as the general case: independent parts that should be presented in order. In an email with attachments, the first part is the body, itself often a multipart/alternative with text and HTML, and each following part is a file, normally marked Content-Disposition: attachment with a file name.
Each part has its own headers and its own transfer encoding, so a PDF part is usually base64 while the text part may be quoted-printable. Gmail limits how many attachments one message may carry and rejects messages over that limit with 552 5.3.4.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2046, section 5.1.3: Mixed Subtyperfc-editor.org
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
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