I do have a short sentence on the homepage saying I used https://github.com/jackbearheart/email-addresses as the validator for the examples. It could be more clearly defined, though.

I see some of the sibling comments seem to imply that "valid" means that it's deliverable at that moment in time, which would make producing a quiz like this some nonsense because then they'd have to actually register xn--tp8h as a domain, and create an MX record for it