So, I would say 40% of my guests do not reply to the welcome message (I always ask a question in the message, too). I contacted one guest on WhatsApp and I told her to check her spam folder but still she found nothing. Then she told me that she realised that when she signed in to booking.com she used an email account which is not her usual one. Some guests do this when they sign up for things on the internet because they are afraid they will be inundated with spam. My only workaround for this situation is to use Whatsapp ad if that fails send an SMS. But it all depends on the guest having given a true contact number and a number which is a mobile phone, not a landline. Any other people have solutions for reaching guests who do not reply to messages we send through booking.com?