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 address2 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.
Message-IDs, if present, are generated by the client program sending the email2 or by the first mail server.3 A common method of generating such ID is by combining the time and domain name, for example: 950124.162336@example.com.4
References
References
- Eoghan Casey (2004). Digital evidence and computer crime: forensic science, computers and the Internet. Academic Press. p. 506. ISBN 0-12-163104-4.
- Resnick, P. (October 4, 2008). Resnick, P (ed.). Internet Message Format. IETF. doi:10.17487/RFC5322. RFC 5322.
- Klensin, J. (October 4, 2008). Simple Mail Transfer Protocol. IETF. doi:10.17487/RFC5321. RFC 5321.
- Levinson, E. (August 4, 1998). Content-ID and Message-ID Uniform Resource Locators. IETF. doi:10.17487/RFC2392. RFC 2392.