Hi,
Please first check the “from” email address used in the settings. Note that if you use a “from” email from a public email service like “@hotmail.com”, “@aol.com”, “@outlook.com”, “@gmail.com”, … it will be filtered by most antispam systems since the email will be identified as “not originated at the expected server”.
That’s in the following settings area:
https://apphourbooking.dwbooster.com/customdownloads/email-settings.png
If that’s OK, then it should be a email delivery issue in the email server. To confirm this I recommend you to install a WordPress email log plugin, these type of plugins will register every email sent from WordPress to the email server and you can detect this way on which side the email is being lost.
Note that the plugin uses the settings specified into the WordPress website to deliver the emails, if your hosting/email service has some specific requirements like a fixed “from” address or a custom “SMTP” server those settings must be configured into the WordPress website.
Thank you for using the plugin!