code: protected function format(MessageInterface $message)
      {
          $headers = array_filter(array_merge(
              $message->getHeaders(),
              ['Reply-To' => $message->getReplyTo()]
          ));
  
          $from = $this->normalizeIdentity($message->getFrom());
  
          $to = [];
          foreach ($this->normalizeIdentities($message->getTo()) as $recipient) {
              $to[] = ['email' => $recipient->getEmail(), 'name' => $recipient->getName(), 'type' => 'to'];
          }
  
          foreach ($this->normalizeIdentities($message->getCc()) as $recipient) {
              $to[] = ['email' => $recipient->getEmail(), 'name' => $recipient->getName(), 'type' => 'cc'];
          }
  
          foreach ($this->normalizeIdentities($message->getBcc()) as $recipient) {
              $to[] = ['email' => $recipient->getEmail(), 'name' => $recipient->getName(), 'type' => 'bcc'];
          }
  
          $tags = [];
          if ($message instanceof TaggableInterface) {
              $tags = (array) $message->getTag();
          }
  
          $metadata = [];
          if ($message instanceof MetadataAwareInterface) {
              $metadata = array_filter($message->getMetadata());
          }
  
          $images = [];
          $attachments = [];
          if ($message instanceof AttachmentsAwareInterface) {
              list($attachments, $images) = $this->processAttachments($message->getAttachments());
          }
  
          $parameters = [
              'key'     => $this->getServerToken(),
              'message' => array_filter([
                  'from_email'  => $from->getEmail(),
                  'from_name'   => $from->getName(),
                  'to'          => $to,
                  'subject'     => $message->getSubject(),
                  'headers'     => $headers,
                  'text'        => $message->getText(),
                  'html'        => $message->getHtml(),
                  'tags'        => $tags,
                  'metadata'    => $metadata,
                  'attachments' => $attachments,
                  'images'      => $images,
              ]),
          ];
  
          if ($this->subaccount) {
              $parameters['message']['subaccount'] = $this->subaccount;
          }
  
          return json_encode($parameters);
      }
docs: {@inheritdoc}