RouteSubscriber.php

<?php

namespace Drupal\mail_login_rpc\Routing;

use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Component\Routing\RouteCollection;

/**
 * Swaps default rpc login controller with ours.
 */
class RouteSubscriber extends RouteSubscriberBase {
  /**
   * {@inheritdoc}
   */
  protected function alterRoutes(RouteCollection $collection) {
    if ($route = $collection->get('user.login.http')) {

    }
  }
}