The Message-ID line gives the message a unique identifier. The Message-ID may not be reused during the lifetime of any previous message with the same Message-ID. (It is recommended that no Message-ID be reused for at least two years.) Message-ID's have the syntax:
<string not containing blank or ">">
In order to conform to RFC-822, the Message-ID must have the format:
where full_domain_name is the full name of the host at which the message entered the network, including a domain that host is in, and unique is any string of printing ASCII characters, not including "<" (left angle bracket), ">" (right angle bracket), or "@" (at sign).
For example, the unique part could be an integer representing a sequence number for messages submitted to the network, or a short string derived from the date and time the message was created. For example, a valid Message-ID for a message submitted from host ucbvax in domain "Berkeley.EDU" would be "<4123@ucbvax.Berkeley.EDU>". Programmers are urged not to make assumptions about the content of Message-ID fields from other hosts, but to treat them as unknown character strings. It is not safe, for example, to assume that a Message-ID will be under 14 characters, that it is unique in the first 14 characters, nor that is does not contain a "/".
The angle brackets are considered part of the Message-ID. Thus, in references to the Message-ID, such as the ihave/sendme and cancel control messages, the angle brackets are included. White space characters (e.g., blank and tab) are not allowed in a Message-ID. Slashes ("/") are strongly discouraged. All characters between the angle brackets must be printing ASCII characters.
RFC1036: [Source:"RFC-1036"] [Last Changed:December 1987] [Copyright: 1987 M. Horton, R. Adams]
RFC-1036:[Previous][Up to Table of Contents] [Next]
For POSTing, modern newsreaders require only "From" "Subject" and "Newsgroups" and will generate the remaining lines. See the note: NNTP-Ext 4.2 The Required Headers in an Article and the POST command
Up to RFC1036 2.1. Required Header lines