According to ICANN rules - especially WDPRS is relevant - every registrar has to ensure, that the whois data of all domains is correct and valid.
Each customer as registrant of a domain, has agreed to this by accepting our, and the according registries Terms and Conditions.
When a registrar - like Joker.com - receives a complaint, he has to start a procedure to either make the owner of a domain correct this whois data, or to prove its correctness.
This is policed by ICANN.
Joker.com is doing this in such a way, that the owner and the admin contact of a domain are provided with an email, asking to fax, email or upload a form, which is attached to this mail, and also available for download at Joker.com (menu 'Support' -> 'Downloads').
If Joker.com does not receive this form within 14 days, the domain will be set on "hold", which means it is not usable any more.
In case you have any questions concerning this, please direct your inquiry to firstname.lastname@example.org.