We track email opens by placing an invisible image inside the emails you send.
If someone responded to your email but ToutApp is not saying it was viewed, then they have not enabled images within their email client.
Some tips for getting better tracking stats on your emails:
1. Include an image in your emails, like a logo, so that your recipient is encouraged to enable images to see your message.
2. Include a link as a call to action in your message.