As soon as I completed installing Exchange 2013 CU1 in my home lab, naturally the very first thing I attempted was sending myself an email, using a simple Telnet, however I was presented with an error:
A bit of searching revealed the answer:
In my home lab environment I currently only have 1 domain controller, so naturally I put my router as my secondary DNS server on all my configurations.
This results in the above error message. I simply removed the secondary DNS which does not point to my internal DNS/Domain Controller and now I am able to receive email.
Forwarding typically would be handled by the DNS servers in your environment, so a secondary public DNS would not be configured in a typical production environment. In a home Lab potentially you might run into this problem.