Dear all,
really sorry for wasting your time, I solved the problem myself: the foreach loop was wrong, and also, I was using $_POST in the wrong way, I corrected as the follow:
foreach ($_POST['_crb_subscription_elements'] as $subscription ) {
$message = sprintf( __( 'Activation Date: %s ' ), $abbonamento['_crb_subscription_activation_date']) . "\r\n\r\n";
wp_mail( $admin_email, sprintf( __( '[From Admin] Subscription activated' ), get_option('blogname') ), $message );
}
Sorry again.