[BCP134]   Best Current Practice 134,
              <https://www.rfc-editor.org/info/bcp134>.
              At the time of writing, this BCP comprises the following:

              Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
              Finch, "Email Submission Operations: Access and
              Accountability Requirements", BCP 134, RFC 5068,
              DOI 10.17487/RFC5068, November 2007,
              <https://www.rfc-editor.org/info/rfc5068>.