protected function createEnvelopeHeaders()
{
$headers = new Horde_Mime_Headers();
$headers->setEOL("\r\n");
$headers->addHeader('From', $this->_getDriver()->getAuth());
$headers->addHeader('To', $this->_getDriver()->getAuth());
$headers->addHeader('Date', date('r'));
$headers->addHeader('Subject', $this->getUid());
$headers->addHeader('User-Agent', 'Horde_Kolab_Storage ' . Horde_Kolab_Storage::VERSION);
$headers->addHeader('MIME-Version', '1.0');
$headers->addHeader('X-Kolab-Type', Horde_Kolab_Storage_Object_MimeType::getMimeTypeFromObjectType($this->getType()));
return $headers;
}