/**
* {@inheritdoc}
*/
protected function setExtHeaders(&$headers, array $data)
{
parent::setExtHeaders($headers, $data);
$headers->addHeaderLine(self::X_GM_MSGID, $data[self::X_GM_MSGID]);
$headers->addHeaderLine(self::X_GM_THRID, $data[self::X_GM_THRID]);
$headers->addHeaderLine(self::X_GM_LABELS, isset($data[self::X_GM_LABELS]) ? $data[self::X_GM_LABELS] : array());
}