>> Domain Mapping problem

1. I have "http://subdomain.domain.com wich is 30.30.32.42 (for ex.)
2. I install domain mapping = Ok
3. I add "http://newdomain.com" in my plesk server, with the same IP : 30.30.32.42 (for ex.)
4. in mapping domain config I try to add "http://newdomain.com" and a redpopup always say
Inacessible domain, what must I do ?
Thank you