MessageID

MessageID, often seen as Message-ID in headers, refers to a unique identifier assigned to a message within a communication system. The purpose of a MessageID is to distinguish each message, support threading and deduplication, and enable reliable tracking across systems and networks. In many contexts, the identifier should remain stable for the lifetime of the message and be globally unique to prevent collisions.

In email, the Message-ID header is defined by the Internet Message Format specification. A typical value appears as a string enclosed in angle brackets, such as <unique-id@domain>. The ID is intended to be globally unique and is usually generated by the sending client or mail transfer agent. While not required by all implementations, a unique Message-ID allows recipients and servers to reference a specific message unambiguously, supports threading through In-Reply-To and References headers, and facilitates message routing and duplicate detection.

Beyond email, MessageID-like identifiers are used in a variety of messaging and logging systems. In message-oriented middleware and messaging protocols, a message identifier may be assigned by the broker or producer and is used to acknowledge delivery, correlate requests and responses, and trace message flows across distributed components. In systems such as JMS and similar architectures, the MessageID (or an equivalent field) assists in auditing, troubleshooting, and ensuring end-to-end visibility.

Generation strategies typically involve a combination of timestamps, unique host or broker identifiers, and random data to ensure low collision probability. Attention to privacy and security is advised, as identifiers can reveal internal topology if exposed publicly.

See also: RFC 5322, In-Reply-To, References, JMS, message tracing.