Wikipedia
Message-ID is a unique identifier for a digital message, most commonly a globally unique identifier used in email and Usenet newsgroups.
Message-IDs are required to have a specific format which is a subset of an email address 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., [email protected]. 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 ( message transfer agents) mangle the ones of Outlook, several same messages may coexist with original and mangled ones and create 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).
Mails to public mailing-lists can be found via their Message-ID by using links from various services:
http://mid.gmane.org/[email protected] http://news.gmane.org/find-root.php?message_id=alpine.BSF.2.00.1004291655110.61392@x.fncre.vasb https://marc.info/[email protected] http://mid.mail-archive.com/[email protected] https://www.freebsd.org/cgi/mid.cgi?db=mid&[email protected] https://lists.debian.org/msgid-search/[email protected] http://lists.debconf.org/cgi-lurker/keyword.cgi?doc-url=/lurker&format=en.html&query=id:[email protected] https://www.w3.org/mid/[email protected] http://www.postgresql.org/message-id/[email protected]