I have EX2013 with the default receive connectors. I had originally setup a hub transport receive connector in order to relay mail from SQL2012 Report Server (http://technet.microsoft.com/en-GB/library/jj657467.aspx) however this causes an issue which periodically stops inbound mail from the internet (see http://social.technet.microsoft.com/Forums/en-US/exchangesvradmin/thread/cd03793b-9e99-474e-8f25-11275652882f). This thread suggests using a Frontend transport receiver, but no matter what configuration or combination of permission and authentication, I cannot get mail to relay through Exchange.
I feel like I'm going around in circles...
