N. Freed and N. Borenstein. <a href="http://www.ietf.org/rfc/rfc2045.txt"><cite>Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies.</cite></a> November 1996. URL: <a href="http://www.ietf.org/rfc/rfc2045.txt">http://www.ietf.org/rfc/rfc2045.txt</a>