code: public ZAuth configurePlain(String domain, String filename)
      {
          Objects.requireNonNull(domain, "Domain has to be supplied");
          Objects.requireNonNull(filename, "File name has to be supplied");
          return send(Mechanism.PLAIN.name(), domain, filename);
      }
docs: Configure PLAIN authentication for a given domain. PLAIN authentication
  uses a plain-text password file. To cover all domains, use "*". You can
  modify the password file at any time; it is reloaded automatically.
  @param domain
  @param filename