Message-ID
A header field holding a globally unique identifier for one message, such as <20261020.4f2a9c@mail.example.com>, used for threading, duplicate detection and tracing.
What it means
RFC 5322 says every message should have a Message-ID and that the identifier must be globally unique. The usual form is a unique left part, an @, and a domain the generating host controls, in angle brackets. It is created by the author's program or, if missing, by the submission server, and replies quote it in In-Reply-To and References to build threads.
Receivers use it for more than threading. Google asks that every message include a valid Message-ID; Gmail rejects messages without one (550 5.7.1) and limits many messages sharing the same Message-ID (421 4.7.28). Google's Feedback Loop documentation also warns against using a per-message value such as the Message-ID as a Feedback-ID identifier.
Example
Message-ID: <20261020091500.4f2a9c@mail.example.com>Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5322, section 3.6.4: Identification Fieldsrfc-editor.org
- RFC 6409, section 8.3: Add 'Message-ID'rfc-editor.org
- Google, Gmail Help: Email sender guidelinessupport.google.com
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
- Google, Gmail Help: Feedback Loopsupport.google.com
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