rfc2822 April 2001

3.6.5. Informational fields

   The informational fields are all optional.  The "Keywords:" field
   contains a comma-separated list of one or more words or
   quoted-strings. The "Subject:" and "Comments:" fields are
   unstructured fields as defined in section 2.2.1, and therefore may
   contain text or folding white space.

subject         =       "Subject:" unstructured CRLF

comments        =       "Comments:" unstructured CRLF

keywords        =       "Keywords:" phrase *("," phrase) CRLF

   These three fields are intended to have only human-readable content
   with information about the message.  The "Subject:" field is the most
   common and contains a short string identifying the topic of the
   message.  When used in a reply, the field body MAY start with the
   string "Re: " (from the Latin "res", in the matter of) followed by
   the contents of the "Subject:" field body of the original message.
   If this is done, only one instance of the literal string "Re: " ought
   to be used since use of other strings or more than one instance can
   lead to undesirable consequences.  The "Comments:" field contains any
   additional comments on the text of the body of the message.  The
   "Keywords:" field contains a comma-separated list of important words
   and phrases that might be useful for the recipient.