[STD72]    Internet Standard 72,
              <https://www.rfc-editor.org/info/std72>.
              At the time of writing, this STD comprises the following:

              Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
              <https://www.rfc-editor.org/info/rfc6409>.