$form_check = mosGetParam($_POST,'form',array());
if(isset($form_check['email'])){ //make sure that your e-mail field id is called \"email\", otherwise change it here
	$avvoid = array('hotmail','yahoo','gmail');//add all the providers you want to filter.
	$found = false;
	foreach($avvoid as $avvoid_key){
		if(strpos($form_check['email'],$avvoid_key)){//make sure that your e-mail field id is called \"email\", otherwise change it here
			$found = true;
		}
	}
 
	if($found) mosRedirect('http://www.link.to.a.content.item.that.says.that.public.emails.are.not.allowed');
}