Magento – How to Get order increment id in an email title in Magento 2

The code which sends the contacts email is:

$post = $this->getRequest()->getPost();
// ... more code ...
$mailTemplate->setDesignConfig(array('area' => 'frontend'))
    ->setReplyTo($post['email'])
    ->sendTransactional(
        Mage::getStoreConfig(self::XML_PATH_EMAIL_TEMPLATE),
        Mage::getStoreConfig(self::XML_PATH_EMAIL_SENDER),
        Mage::getStoreConfig(self::XML_PATH_EMAIL_RECIPIENT),
        null,
        array('data' => $postObject)
    );

You can see, it uses the value email which was posted from the contacts form, which actually is exactly what you want. Some modification you or an extension you installed must have changed the default behavior.

You have to pass \Magento\Sales\Api\OrderRepositoryInterface in construct of your class.

protected $orderRepository;

public function __construct(
    ..., 
    \Magento\Sales\Api\OrderRepositoryInterface $orderRepository
){
    ...
    $this->orderRepository = $orderRepository;
}

Then you can do following:

$order = $this->orderRepository->get($orderId);
$orderIncrementId = $order->getIncrementId();