code: private function buildHeaders($required_headers = []) {
      $build_headers = array_merge($this->headers, $required_headers);
      $headers = [];
      foreach ($build_headers as $name => $value) {
        $headers[] = "{$name}: {$value}";
      }
      return implode("\r\n", $headers)."\nThis is a multi-part message in MIME format.\n";
    }
docs: Create message headers.
  
  @access private
  @param array $required_headers Any headers that should append/replace the defined headers.
  @return string The headers.