How to send email using Rules from user references CCK Field?
In the recipient input, try to write a php code which returns the email :

$customer_node = node_load( $node-&gt;field_customer['und'][0]['nid'] );
$account = user_load( $customer_node-&gt;field_user['und'][0]['uid'] );
print $account-&gt;mail;


Note that a better way is to use a custom function in a custom module :

print _get_customer_email( $node );


and

function _get_customer_email( $node ){
  $customer_node = node_load( $node-&gt;field_customer['und'][0]['nid'] );
  $account = user_load( $customer_node-&gt;field_user['und'][0]['uid'] );
  return $account-&gt;mail;
}