Inside your test script you can create javascript functions in order to generate randomly the email address.
casper.then(function() {
this.echo('signing up with the follow credentials:', 'COMMENT');
this.fill('.sign_up', {
'name': 'mariah carey',
'email': generatePassword(),
'password': 'testtest123'
}, true);
}
...
function generatePassword(){
var email;
// Here code for generate email
return email;
}
You can do the same for name and password.