Sometime during the night (in our part of the world) the ClamAV daemon process died on our outbound mail server. Since we scan all submitted mail submitted for viruses, this was causing a "451 4.3.0 Error: queue file write error" to be returned. Affected customers (from the logs) will receive a service credit for this outage.
Since this is the first time the Clam daemon has ever died on us, we didn't have that process in the monitoring system. Rest assured that we will be adding it now. We restarted the process, and our outbound mail has returned to normal.