Hi,
Try first using a “from” email address that belongs to your website domain (example something @christophefassi.fr), this is the most common restriction applied in most hosting services.
If that doesn’t work please check if your hosting service requires some specific configuration to send emails from PHP/WordPress websites. The plugin uses the settings specified into the WordPress website to deliver the emails, if your hosting has some specific requirements like a fixed “from” address or a custom “SMTP” server those settings must be configured into the WordPress website.
For a more detailed assistance you can open a ticket at https://apphourbooking.dwbooster.com/contact-us , we offer free support for this plugin.
Thank you for using the plugin!