Many links in different post messages like http://www.site.com/www.example.com
this is most likely caused by urls being posted without the leading protocol specification (http://) which are treated by the browser as relative urls. you could fix this by editing or processing the posted urls.
otherwise you should use mod_rewrite to redirect these requests to the proper hostname. the difficulty is reliable recognizing these external urls so they can be captured for the back reference in the RewriteRule (while excluding local urls from redirecting).