Glossary · Protocols
Message-ID is a header holding a globally unique identifier for an email, used for threading replies and spotting duplicates.
RFC 5322 defines it as a unique value in angle brackets, conventionally a random part, an @, and the sender's domain. Replies reference it in In-Reply-To and References, which is how mail apps build conversation threads.
A missing or malformed Message-ID breaks threading and is a spam signal; Gmail rejects some messages that lack one. Mail servers and sending libraries normally add it for you.
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