User does not receive email after booking an appointment

Hi,
I am trying to use your plugin in a project. Everything is working fine except one feature. I checked the option to send user email when he/she fills up the form. However, the user does not receive an email when he/she books an appointment. But admin or worker gets email properly. So, email feature is enabled on my server. And I am not using any extra plugins that could conflict. Extra plugins used in my site are, contact form 7, MailChimp for WordPress and KingComposer. I have also attached the link above where I am testing your plugin. you can book a test appointment and see if you can receive an email. Please fix this issue or please tell me which file I should look into to find the code where email sending works. I am a developer myself, hopefully, I can fix this.

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