A typical FAQ about URL rewriting is how to redirect failing requests on webserver A to webserver B. Usually this is done via ErrorDocument CGI-scripts in Perl, but there is also a mod_rewrite solution. But notice that this performs more poorly than using an ErrorDocument CGI-script!

Place this in your httpd.conf og .htaccess file on the domain you want to redirect HTTP requests from
Solution
This is one possible solution but less flexible, and is less error safe

RewriteEngine on
RewriteCond   /your/docroot/%{REQUEST_FILENAME} !-f
RewriteRule   ^(.+)                             http://webserverB.dom/$1

The problem here is that this will only work for pages inside the DocumentRoot.

Source