Zoho Mail Lite - Contact Email address should not be the same to the Domain Name that will be use for a free trial.

Zoho Mail Lite - Contact Email address should not be the same to the Domain Name that will be use for a free trial.

Why it has to be be other domain address? I tried putting my gmail address It worked, it can proceed. But When I input the other department email address but the same domain name in the contact email fields it rejects. May I asked what is the reason that it has to be another email domain address?