[BCP167]   Best Current Practice 167,
              <https://www.rfc-editor.org/info/bcp167>.
              At the time of writing, this BCP comprises the following:

              Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
              Mailing Lists", BCP 167, RFC 6377, DOI 10.17487/RFC6377,
              September 2011, <https://www.rfc-editor.org/info/rfc6377>.