A message ID is a globally unique identifier to an e -mail message or a Usenet postings.
The message ID is generated either by sending e -mail or news reader, or use the SMTP or NNTP server. Usually it consists of a domain name and a domain- wide unique identifier ( random string (?) ), Separated by an @ sign. This same message IDs in building e- mail addresses, but with which it can not be confused. In order not to compromise the uniqueness, the message ID should only be generated by e- mail or Usenet program if a separate domain name is available.
In a message is its own message ID in the same header row. The Message-ID of the message to which the message relates, are in the References header line. Using this information, an e- mail or Usenet news program to the corresponding answers are grouped ( Threading).
Google Groups allows you to search Usenet posts with a given message ID under " Advanced Groups Search " and then "Find posts with Entry ID".
For identification of Message - IDs of Usenet posts them is "news: " prefix.
The header line message ID is mentioned in RFC 724.