public function send(): Letter
    {
        $options = [
            'headers' => [
                'Content-Source' => $this->getEndpointMailbox().'/letters/'.$this->getLetterId(),
            ],
        ];

        if ($this->getEnvelope()->isHybridLetter() && null !== $this->getDeliveryOptions()) {
            $options['headers']['Content-Type'] = $this->getDeliveryOptions()->getMimeType();
            $options['json']                    = $this->getDeliveryOptions();
        }

        $this->getHttpClientForSend()->request('POST', '/deliveries', $options);

        return $this;
    }