MIME boundary
The string, declared in a multipart Content-Type, that marks where each part of a MIME message starts and where the last one ends.
What it means
A multipart Content-Type carries boundary="...", a string of 1 to 70 characters chosen so that it does not occur in the content. Each part begins on a line made of two hyphens and the boundary, and the end of the last part is marked by the boundary followed by two more hyphens. Text before the first boundary, the preamble, is ignored by MIME readers.
Boundaries are easy to get wrong in hand-built mail: a boundary string that appears inside a part, a missing closing line, or a mismatch between header and body all produce messages that display partly or not at all. Changing boundaries in transit also breaks DKIM, and Google lists modifying MIME boundaries among the things that make forwarded mail fail it.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2046, section 5.1.1: Common Syntaxrfc-editor.org
- Google, Gmail Help: Best practices for forwarding email to Gmailsupport.google.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes