The Subject line (formerly Title) tells what the message is about. It should be suggestive enough of the contents of the message to enable a reader to make a decision whether to read the message based on the subject alone. If the message is submitted in response to another message (e.g., is a follow-up) the default subject should begin with the four characters "Re:", and the References line is required. For follow-ups, the use of the Summary line is encouraged.
RFC1036: [Source:"RFC-1036"] [Last Changed:December 1987] [Copyright: 1987 M. Horton, R. Adams]
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Up to RFC1036 2.1. Required Header lines