code: def check_connection (self):
          
          for mail in sorted(self.addresses):
              self.check_smtp_domain(mail)
              if not self.valid:
                  break
docs: Verify a list of email addresses. If one address fails,
          the whole list will fail.
  
          For each mail address the MX DNS records are found.
          If no MX records are found, print a warning and try
          to look for A DNS records. If no A records are found either
          print an error.