Exchange 2013: 421 4.3.2 Service not available

Today I had an issue with a new Exchange 2013 deployment, which had the CAS and MBX role collocated. After about 8-12 hours my Exchange 2013 server will respond with;

Trying x.x.x.x...
Connected to x.x.x.x.
Escape character is '^]'.
421 4.3.2 Service not available
Connection closed by foreign host.

And in the SMTP protocol logs i would have the following - 421 4.3.2 Service not available all of a sudden;

When i created my Internet Receive connector - I selected the Hub Transport Role which is incorrect:

When creating a Internet Receive connector, you should assign the receive connector to the Frontend Transport Role;















When i read the Mail Flow explained article on Exchange 2013 it became apparent i made a mistake (http://technet.microsoft.com/en-us/library/aa996349(v=exchg.150).aspx)

• Front End Transport service   This service runs on all Client Access servers and acts as a stateless proxy for all inbound and outbound external SMTP traffic for the Exchange 2013 organization. The Front End Transport service doesn't inspect message content, only communicates with the Transport service on a Mailbox server, and doesn't queue any messages locally.

Hopefully this helps people who made the same mistake as me.