Message-ID is a unique identifier for a digital message, most commonly a globally unique identifier used in email and Usenet newsgroups.^[1]

This article may be too technical for most readers to understand. (June 2020)

Message-IDs are required to have a specific format which is a subset of an email address^[2] and be globally unique. No two different messages must ever have the same Message-ID. If two messages have the same Message-ID, they are assumed to be the same and one version is discarded. This can cause issues if tools mangle the IDs created by other tools. Such a problem has been reported with Google MTAs mangling Message-IDs created by Outlook, making it difficult to reference other messages and breaking threading.^{[citation needed]}

Message-IDs, if present, are generated by the client program sending the email^[2] or by the first mail server.^[3] A common method of generating such ID is by combining the time and domain name, for example: [email protected].^[4]