References header
The header field in a reply listing the Message-IDs of earlier messages in the conversation, used by mail programs to rebuild a thread.
What it means
Under RFC 5322 a reply's References field is the parent's References (or, if it has none, its single In-Reply-To ID) followed by the parent's own Message-ID, so the list grows by one ID at each step of the conversation. Programs use it to reconstruct a thread even when some messages in it are missing.
Gmail documents grouping automated mail into one conversation when messages have the same recipients, senders or subject, share reference header IDs, and arrive within a week of each other. To keep notifications apart, Google suggests a unique reference value or a new subject per message; follow-up sequences do the opposite and keep References pointing at the first message.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5322, section 3.6.4: Identification Fieldsrfc-editor.org
- Google, Gmail Help: Group emails into conversationssupport.google.com
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