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., 950124.162336@example.com. 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/alpine.BSF.2.00.1004291655110.61392@x.fncre.vasb http://news.gmane.org/find-root.php?message_id=alpine.BSF.2.00.1004291655110.61392@x.fncre.vasb https://marc.info/?i=Prayer.1.0.18.0811191202370.4107@sabinus.cs.umn.edu http://mid.mail-archive.com/9dc4201d0807272359r1f0ad9d6ta44f31f439de58b7@mail.gmail.com https://www.freebsd.org/cgi/mid.cgi?db=mid&id=199802242058.MAA24843@monk.via.net https://lists.debian.org/msgid-search/20050606213954.GC884@finlandia.infodrom.north.de http://lists.debconf.org/cgi-lurker/keyword.cgi?doc-url=/lurker&format=en.html&query=id:87vey4ogcj.fsf@vorlon.ganneff.de https://www.w3.org/mid/alpine.BSF.2.00.1004291655110.61392@x.fncre.vasb http://www.postgresql.org/message-id/20060419051850.96025.qmail@web31802.mail.mud.yahoo.com