code: protected function setRecipient(MailBuilderInterface $mailBuilder)
      {
          $availabilityAlertSubscriptionTransfer = $mailBuilder->getMailTransfer()
              ->requireAvailabilityAlertSubscription()
              ->getAvailabilityAlertSubscription();
  
          $mailBuilder->addRecipient(
              $availabilityAlertSubscriptionTransfer->getEmail(),
              ''
          );
  
          return $this;
      }
docs: @param \Spryker\Zed\Mail\Business\Model\Mail\Builder\MailBuilderInterface $mailBuilder
  
  @return $this