Message-ID is a globally unique-style identifier associated with a message.
Message-ID: <20261003.abc123@example.com>It is extremely useful when correlating a message across client, gateway and server logs.
What it does not prove
Message-ID is message content, not a cryptographic proof of origin. A malicious sender can choose a value, and some systems can generate or replace missing identifiers.
Use it as a correlation key together with trusted logs, timestamps and other evidence.