In-Reply-To header
The header field in a reply that holds the Message-ID of the message being answered, so mail programs can link the two.
What it means
RFC 5322 says replies should carry In-Reply-To with the parent message's Message-ID (or several, when one reply answers more than one message), together with a References field. Mail programs use the pair to put a reply in the right conversation, and Gmail groups messages that share reference IDs.
Tools that send follow-ups in the same thread set In-Reply-To and References to the earlier message's ID, which is what keeps a sequence threaded for the recipient. A message that only adds Re: to its subject has neither field, and Google asks senders not to use Re: in subjects unless the message is a real reply.
Example
In-Reply-To: <20261020091500.4f2a9c@mail.example.com>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
- Google, Gmail Help: Email sender guidelinessupport.google.com
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