[BCP138]   Best Current Practice 138,
              <https://www.rfc-editor.org/info/bcp138>.
              At the time of writing, this BCP comprises the following:

              Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
              Mail System Status Codes", BCP 138, RFC 5248,
              DOI 10.17487/RFC5248, June 2008,
              <https://www.rfc-editor.org/info/rfc5248>.