I am facing the same problem. Have SBG 8.0.3-11 configured for inbound mail relay.
The number of connections from a single IP address is set as default say 2000.
But the problem is not with acceptance from the sender. The issue that we are observing is that after being accepted by SBG the scanner is trying to relay the mail downstream but that does not happen.
Moreover the number of connections per unit time from the sender MTA of the domain is very negligible. There is no way the threshold could have been exceeded.
Further more, the mail is lost as it is not even retained in the delivery queues.
Can you please help?