Good evening Benjamin and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the emails not being received while using Event Tickets Plus.
I would love to help you with this topic.
It is most likely that something is blocking the wp_mail on your install.
The fastest way around this is to install and configure a plugin that sends emails from your site through a legit email address, here is one of them: https://wordpress.org/plugins/easy-wp-smtp/
Let me know how that goes.
Best regards,
Geoff B.