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

Message-IDs are required to have a specific format which is a subset of an email address[2] (though Microsoft Outlook is known to generate invalid IDs with three parts[3]) and to be globally unique. That is, no two different messages must ever have the same Message-ID. A common technique used by many message systems is to use a time and date stamp along with the local host's domain name, e.g.,[4]

Message-IDs, if present, are generated by the client program sending the email (mail user agent, or MUA) or by the first mail server (mail submission agent, or MSA).[5]

A large number of mails to public mailinglists can be found via their Message-ID by using a link of the form

See also[edit]