WP send mail notification to new user created programmatically

I have a script to create a new wordpress user by checking the email and password in a external database. I have a script in place to enable login in with the email address.

The script to add the users from the external database works as intend, but the notification emails don’t.

I’ve tried (the password is stored in plain text in the external db – I know it’s not recommended at all):

wp_new_user_notification($new_user_id, $result->det->USR_PASSWORD);
wp_new_user_notification($user->ID, $result->det->USR_PASSWORD);
wp_new_user_notification($user->data->ID, $result->det->USR_PASSWORD);

Nothing seems to work. Here is the full script I use:

add_filter('authenticate', 'uj_auth', 20, 3);

function uj_auth( $user, $username, $password ){
 if ( is_a($user, 'WP_User') ) { return $user; } //check if existing user
 if ( empty($username) || empty($password) ) {
  $error = new WP_Error();
  if ( empty($username) )
    $error->add('empty_username', __('<strong>ERROR</strong>: The username field is empty.'));
  if ( empty($password) )
    $error->add('empty_password', __('<strong>ERROR</strong>: The password field is empty.'));
return $error;
 }

$userdata = get_user_by('login', $username);

if ( !$userdata ): //if not an existing user

$ch = curl_init();

curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_URL,"http://webstite.com");
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_FORBID_REUSE, true);
curl_setopt($ch, CURLOPT_FRESH_CONNECT, true);
curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);  
curl_setopt($ch, CURLOPT_POSTFIELDS,"apikey=apikey&username=$username&password=$password");

$json = curl_exec ($ch);
curl_close ($ch); 
$result = json_decode($json); //the result array from the external api

if ($result->result == 'OK'): //if user and pass match those in database result is OK
    //Verify if the username is not NULL. User user-id instead
    if(is_null($result->det->USR_USERNAME)) $result->det->USR_USERNAME = $result->det->USR_ID;
  // Setup the required user information
    $userdata = array( 
          'user_email' => $result->det->USR_EMAIL,
          'user_pass' => $result->det->USR_PASSWORD,
          'user_login' => $result->det->USR_USERNAME,
          'first_name' => $result->det->USR_PRENUME,
          'last_name' => $result->det->USR_NUME
          );
  // A new user has been created
    $new_user_id = wp_insert_user( $userdata );
  // Load the new user info
    $user = new WP_User ($new_user_id);

  // Email the user
    wp_new_user_notification($new_user_id, $result->det->USR_PASSWORD);

   else:

  return new WP_Error('invalid_username', sprintf(__('<strong>ERROR</strong>: Utilizator și/sau parolă introduse greșit sau inexistente. <a href="'.site_url('inregistrare').'" title="Inregistreaza-te">Inregistrează-te!</a>')));

   endif;

else:

$userdata = apply_filters('wp_authenticate_user', $userdata, $password);
if ( is_wp_error($userdata) )
    return $userdata;

if ( !wp_check_password($password, $userdata->user_pass, $userdata->ID) )
    return new WP_Error( 'incorrect_password', sprintf( __( '<strong>ERROR</strong>: The password you entered for the username <strong>%1$s</strong> is incorrect. <a href="%2$s" title="Password Lost and Found">Lost your password</a>?' ),
    $username, wp_lostpassword_url() ) );

$user =  new WP_User($userdata->ID);

endif;

//remove_action('authenticate', 'wp_authenticate_username_password', 20);

return $user;

}

So what am I doing wrong?