[STD10]    Internet Standard 10,
              <https://www.rfc-editor.org/info/std10>.
              At the time of writing, this STD comprises the following:

              Postel, J., "Simple Mail Transfer Protocol", STD 10,
              RFC 821, DOI 10.17487/RFC0821, August 1982,
              <https://www.rfc-editor.org/info/rfc821>.

              Partridge, C., "Mail routing and the domain system",
              STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986,
              <https://www.rfc-editor.org/info/rfc974>.

              Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
              DOI 10.17487/RFC1869, November 1995,
              <https://www.rfc-editor.org/info/rfc1869>.

              Klensin, J., Freed, N., and K. Moore, "SMTP Service
              Extension for Message Size Declaration", STD 10, RFC 1870,
              DOI 10.17487/RFC1870, November 1995,
              <https://www.rfc-editor.org/info/rfc1870>.