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; }