The default installation of Exchange 2013 will allow an organization to receive emails from the internet. This is a shift from the setting we have in Exchange 2010.
An out of the box installation of a 2013 CAS has a receive connector named “Default FrontEnd <CAS Server Name>.
This connector is configured to accept anonymous SMTP connections from other mail servers in the internet. This means that no additional configuration is required to make your inbound email flow work!