What is the “Message-ID”?

The Message-ID is the unique fingerprint of an email. It is a globally unique identifier that helps mail systems track and differentiate each message, much like a ship’s registration number in a harbor. Every proper email sent through SMTP should have one, and it typically looks like ``.

Behind the scenes, this ID helps prevent duplication, threading errors, and confusion when replies and forwards start flowing back and forth. Some servers generate it automatically; others rely on the sending client to do so. If it’s missing or malformed, it can raise deliverability red flags.