[STD85]    Internet Standard 85,
              <https://www.rfc-editor.org/info/std85>.
              At the time of writing, this STD comprises the following:

              Hansen, T., Ed. and A. Melnikov, Ed., "Message Disposition
              Notification", STD 85, RFC 8098, DOI 10.17487/RFC8098,
              February 2017, <https://www.rfc-editor.org/info/rfc8098>.