rfc1036 December 1987

2.1.4.  Subject

    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.