Good evening Aga and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the tickets not all being sent properly.
I would love to help you with this topic.
In all likelihood, this is caused by some form of spam filtering.
When that happens, the best solution is to have the system send tickets from any chosen email address by simply installing, configuring and testing the following plugin: https://wordpress.org/plugins/easy-wp-smtp/
Using a real SMTP server with authentication will bypass the built-in WP mail functionality altogether.
Please make sure you configure and test the settings using the plugin’s built-in tool.
Let me know how that goes.
Best regards,
Geoff B.