How to redirect all 404s to the same path on a different server
Here is small trick:

Enable module PHP Filter, add PHP filter to administrator

Create a node can use php filter, add code: (We are using header here, not use drupal_goto because handle 404 workfollow of Drupal will redirect to the node first.

&lt;?php
  $url = drupal_get_destination();
  header('Location: http://other.example.com/' . $url['destination']);
  drupal_exit();
?&gt;


Config Drupal using 404 page is the node you was create.