Great post written by John Bailey where where the author explains the problem of enabling recipient filtering on the backend servers.
With the arrival of Exchange 2013 came some design changes and, with those changes, inevitably something was bound to catch us off-guard. To help explain this better, enter the proxy from the CAS Frontend to the Mailbox backend.
All SMTP connections (we’re focusing on SMTP for this post) utilize the proxy of the Frontend Transport to make an SMTP connection. Even on multi-role servers, the incoming request on 25 utilizes the proxy to 2525 for Transport.
Read more at source: http://blogs.technet.com/b/johnbai/archive/2014/08/06/exchange-2013-recipient-filtering-agent-more-lessons-in-proxy.aspx