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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] On the other hand, if two messages have the same Message-ID, they are presumed to be the same one and one version is discarded. But, as Google MTAs mangle the badly formed ones of Outlook, several same messages may coexist with original and mangled ones and indescribable problems in referencing each other for threading.

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]