This field lists the message ID's of any articles prompting the submission of this article. It is required for all follow-up articles, and forbidden when a new subject is raised. Implementations should provide a follow-up command, which allows a user to post a follow-up article. This command should generate a Subject line which is the same as the original article, except that if the original subject does not begin with "Re: " or "re: ", the four characters "Re: " are inserted before the subject. If there is no References line on the original header, the References line should contain the message ID of the original article (including the angle brackets). If the original article does have a References line, the followup article should have a References line containing the text of the original References line, a blank, and the message ID of the original article.

The purpose of the References header is to allow articles to be grouped into conversations by the user interface program. This allows conversations within a newsgroup to be kept together, and potentially users might shut off entire conversations without unsubscribing to a newsgroup. User interfaces may not make use of this header, but all automatically generated followups should generate the References line for the benefit of systems that do use it, and manually generated followups (e.g. typed in well after the original article has been printed by the machine) should be encouraged to include them as well.

The References header lists optionally CFWS-separated message identifiers of precursors. The content syntax makes use of syntax defined in [RFC 2822].

      References-content  = msg-id *( CFWS msg-id )

        NOTE: This differs from the syntax of [RFC 2822] by requiring at
        least one CFWS between the msg-ids (this was an [RFC 1036]
        requirement).

A followup MUST have a References header, and an article that is not a followup MUST NOT have a References header. In a followup, if the precursor did not have a References header, the followup's References-content MUST be formed by the message identifier of the precursor. A followup to an article which had a References header MUST have a References header containing the precursor's References- content (subject to trimming as described below) plus the precursor's message identifier appended to the end of the list (separated from it by CFWS).

   Followup agents SHOULD NOT trim message identifiers out of a
   References header unless the number of message identifiers exceeds
   21, at which time trimming SHOULD be done by removing sufficient
   identifiers starting with the second so as to bring the total down to
   21 (but the first message identifier MUST NOT be trimmed). However,
   it would be wrong to assume that References headers containing more
   than 21 message identifiers will not occur.

      References: <i4g587y@site1.example>
      References: <i4g587y@site1.example> <kgb2231+ee@site2.example>
      References: <i4g587y@site1.example> <kgb2231+ee@site2.example>
         <222@site1.example> <87tfbyv@site7.example>
         <67jimf@site666.example>
      References: <i4g587y@site1.example> <kgb2231+ee@site2.example>
         <tisjits@smeghead.example>