public function addRecipientMetadata($recipient, $data)
    {
        foreach ($this->recipientMetadata as $idx => $rcptMetadata) {
            if (isset($rcptMetadata['rcpt']) && $rcptMetadata['rcpt'] == $recipient) {
                if (is_array($data)) {
                    foreach ($data as $k => $v) {
                        $this->recipientMetadata[$idx]['values'][$k] = $v;
                    }
                }
                else {
                    $this->recipientMetadata[$idx]['values'][] = $data;
                }
                return $this;
            }
        }

        $this->recipientMetadata[] = array('rcpt' => $recipient, 'values' => is_array($data) ? $data : array($data));

        return $this;
    }