Seems to send but only intermittently received

Hi there. I have made a reservation form on this website over a year ago. Over 3,000 form submissions were made since then. Can I assume all “entries” were successfully sent? If not, would it indicate under “entries” if they were not sent?

The issue is that my client receives almost all the submissions but every once in a while (6 times this last week) they did not receive them. Should I suspect Caldera or the email provider?

The page I need help with: [log in to see the link]