A part of what once was the Hub Transport Service has been moved to the Exchange 2013 Client Access Server under the name of the Frontend Transport Service.

This service is responsible for proxying inbound traffic to and (if configured) outbound SMTP traffic from your Exchange organization while maintaining the stateless nature of the server role. Despite the fact that SMTP components on the Client Access Server are running at Layer 7, no mail is ever stored or queued locally on the server. As a result, the Client Access Server will not perform actions such as content filtering or attachment scanning. These tasks are set aside for the Mailbox Transport Service which is now part of the Mailbox Server role.

In theory you would expect the service to be able to handle other tasks which do not involve (temporarily) storing messages locally for example, connection filtering. Sadly, that feature somehow didn't make it into the current version of the product. Hopefully we will see...