Hello Jesper
That mean mail settings on your site is wrong. Please go to System -> Global Configuration, look at Server Tab, Mail Settings section and correct it
Once it's corrected (you might have to contact your hosting provider and ask them to see what parameters to use for Mail Settings, you can test it again by access to Users -> Manage, create a new user account. If that user receives email, that means Mail Settings is correct and it should work fine in Events Booking
Regards,
Tuan
