This field lists the Message-ID's of any messages prompting the submission of this message. It is required for all follow-up messages, and forbidden when a new subject is raised. Implementations should provide a follow-up command, which allows a user to post a follow-up message. This command should generate a Subject line which is the same as the original message, 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 message (including the angle brackets). If the original message does have a References line, the follow-up message should have a References line containing the text of the original References line, a blank, and the Message-ID of the original message.
The purpose of the References header is to allow messages 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 need not make use of this header, but all automatically generated follow-ups should generate the References line for the benefit of systems that do use it, and manually generated follow-ups (e.g., typed in well after the original message has been printed by the machine) should be encouraged to include them as well.
It is permissible to not include the entire previous References line if it is too long. An attempt should be made to include a reasonable number of backwards references.
RFC1036: [Source:"RFC-1036"] [Last Changed:December 1987] [Copyright: 1987 M. Horton, R. Adams]
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Up to RFC1036 2.2. Optional Headers