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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] 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:[4]


  1. ^ Eoghan Casey (2004). Digital evidence and computer crime: forensic science, computers and the Internet. Academic Press. p. 506. ISBN 0-12-163104-4.
  2. ^ a b Resnick, P. (October 4, 2008). Resnick, P (ed.). "Internet Message Format". RFC Series. doi:10.17487/RFC5322 – via
  3. ^ Klensin, J. (October 4, 2008). "Simple Mail Transfer Protocol". RFC Series. doi:10.17487/RFC5321 – via
  4. ^ Levinson, E. (August 4, 1998). "Content-ID and Message-ID Uniform Resource Locators". RFC Series. doi:10.17487/RFC2392 – via

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