I'm looking for something that can't be gamed by the botters.
Mail to all users - lottery (depending on prize fund) split between ID ranges - and make it one entrance per email reply.
Reply with IP address, last roll Server / Client Seeds etc
With a XX hr reply time frame.
You'll be able to match the data - Plus
Botters will have to scrawl through their email boxes they generated, and provide you with IP ranges for blocking 😉
AND you know that your mailing list is up-to-date.
(Helps with Euro/UK GDPR rule's should you be approached)