the file xml-rpc.php is a wordpress file that is not used by most plugins or sites and is a vector that some hackers use to try to exploit or attack your site. Due to a large number attempts against this file across many sites, We have chose to block access to this file server wide. Now if you do find you have a need for the file (a plugin called jetpack is the most likely thing that uses it, and its also used for pingbacks which really are not useful anymore) you can add this to your .htaccess file to re enable it for your site (or email us we will do it for you):

<Files xmlrpc.php>
  Order Deny,Allow
  Allow from all
</Files>