Message-ID is a

unique identifier A unique identifier (UID) is an identifier that is guaranteed to be unique among all identifiers used for those objects and for a specific purpose. The concept was formalized early in the development of computer science and information systems. ...

for a digital message, most commonly a

globally unique identifier A universally unique identifier (UUID) is a 128-bit label used for information in computer systems. The term globally unique identifier (GUID) is also used. When generated according to the standard methods, UUIDs are, for practical purposes, uni ...

used in

email Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic ( digital) version of, or counterpart to, mail, at a time when "mail" mea ...

and

Usenet newsgroup A Usenet newsgroup is a repository usually within the Usenet system, for messages posted from users in different locations using the Internet. They are discussion groups and are not devoted to publishing news. Newsgroups are technically distinc ...

s. Message-IDs are required to have a specific format which is a subset of an

email address An email address identifies an email box to which messages are delivered. While early messaging systems used a variety of formats for addressing, today, email addresses follow a set of specific rules originally standardized by the Internet Enginee ...

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 email or by the first mail server. A common method of generating such ID is by combining the time and domain name, for example: [email protected].

References

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External links

Recommendations for generating Message IDs

MessageID-Finder Thunderbird extension

ThunderLink Thunderbird extension
Unique identifiers Email Usenet