How I redirect only 404 pages to a new (sub) domain with htaccess in wordpress?

See more:

Apache

redirection

htaccess

Hi,

For a client I want to do the following thing with .htaccess:

domainname.com/path must redirect to sub.domainname.com/path, but only if the first page is a deleted page / 404 page.

So all the existing pages must stay intact, and the deleted pages must redirect to a subdomain with the same path.

I used the following code on a Wordpress website, to only redirect 404 error pages to a new domain:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain\.com$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d 
RewriteRule ^ http://example.com%{REQUEST_URI} [L,R=301]

I have overwritten the following code in the existing htaccess file:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

It is not working. The problem is that now ALL pages are redirected to the new domain.

Only the 404's must be redirected.

What do I wrong?

Thanks!

What I have tried:

Tried different htaccess codes