What Is a Message-ID?

A Message-ID is a unique identifier that the sending system gives each message. It appears in angle brackets with a left part and a domain, and mail programs use it to link replies into threads and to avoid showing duplicates.

Format

Message-ID: <[email protected]>
               ^ unique part        ^ domain of the generating system

The text before the @ is chosen by the system that created it, often a timestamp, counter or random value. The part after the @ is typically the hostname or domain of that system. Square-bracket IP literals are allowed but unusual.

How it is used

• Replies carry the original ID in In-Reply-To and References, which is how clients build conversation threads.
• Mail servers use it to detect duplicate deliveries.
• Administrators search server logs for it to follow one message across systems.

Reading the domain part

The domain tells you which system generated the ID, which is often the sender's mail platform. It is common for it to differ from the From domain when a marketing or transactional service sends on behalf of someone. A domain such as localhost, or a name without a dot, suggests a misconfigured or custom sender, which is an observation rather than a verdict.

Anomalies worth noting

• Missing Message-ID (most mail has one; receivers often add it).
• No angle brackets, whitespace or more than one @.
• More than one Message-ID header in a single message.
• An IP literal or a non-qualified domain on the right-hand side.

The Email Header Analyzer parses the Message-ID, shows its domain part and format, and flags duplicates or unusual structure. Because IDs can be used to look up a message in logs, treat them as sensitive and remove them with the Log Sanitizer before you share headers.