The application event log will contain event log entries ID 7012, generated by the MSExchangeFrontEndTransport, mentioning that “The service state for frontend transport is inconsistent. Current state – Inactive. Expected state – Active”:
When inspecting the component state from the Exchange Management Shell using:
Get-ServerComponentState <ServerID> -Component FrontendTransport
you will notice that it really is inconsistent, as Exchange will report that the component is active:
The quick workaround for this issue at the moment is to restart the Frontend Transport service:
After a restart of the service, or system restart if you must, the component state is working fine again and connections are accepted. In addition, the MSExchangeFrontendTransport will generate an event log entry ID 7009, “Retrieved the service state. Host service – FrontendTransport, Service state data – Active.”
Exchange and The UC Architects fellow Paul Cunningham discovered the same issue and blogged about it here.