[STD60]    Internet Standard 60,
              <https://www.rfc-editor.org/info/std60>.
              At the time of writing, this STD comprises the following:

              Freed, N., "SMTP Service Extension for Command
              Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920,
              September 2000, <https://www.rfc-editor.org/info/rfc2920>.