Sorry my mistake, didn't understand your question.
p3p.sytes.net is not a free domain but a free subdomain of sytes.net Therefore you can not register it. Go back to where you got your free subdomain and see if you can get another called mail.p3p.sytes.net. I think you can have a subdomain with a dot in it, ie mail.p3p, if not try to get mailp3p
Hope this helps