Message-IDs are required to have a specific format which is a subset of an email address (though Microsoft Outlook is known to generate invalid IDs with three parts) 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@example.com.
Message-IDs are generated by the server sending the email. 
A large number of mails to public mailinglists can be found via their Message-ID by using a link of the form http://mid.gmane.org/alpine.BSF.firstname.lastname@example.org
- RFC 822 (superseded by 2822)
- RFC 2111 Content-ID and Message-ID Uniform Resource Locators
- RFC 2822 (superseded by 5322)
- RFC 5322 Internet Message Format
- Recommendations for generating Message IDs
- Eoghan Casey (2004). Digital evidence and computer crime: forensic science, computers and the Internet. Academic Press. p. 506. ISBN 0-12-163104-4.
- RFC5322 - Internet Message Format
- SpamAssassin Bug 5666 - New Outlook Message-ID format causes FORGED_MUA_OUTLOOK FP
- RFC2392 - Content-ID and Message-ID Uniform Resource Locators
- GMail - Reading full email headers
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