public function addSignature(Swift_Mime_HeaderSet $headers)
{
$header = new Swift_Mime_Headers_OpenDKIMHeader('DKIM-Signature');
$headerVal = $this->_dkimHandler->getSignatureHeader();
if (!$headerVal) {
throw new Swift_SwiftException('OpenDKIM Error: ' . $this->_dkimHandler->getError());
}
$header->setValue($headerVal);
$headers->set($header);
return $this;
}