451 4.4.0 Primary target IP address responded with:” 421 4.7.0 too many errors on this connection, closing transmision channel.” Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failover

Solution: The error 4.7.0 with 451 4.3.1 "Insufficient System Resources" signifies that the inbound mail submission has stopped due to resource constraint such as disk space being low on the volume where the queue database is located. The following changes are required on EdgeTransport.exe.config file to disable the "BackPressure". This is not recommended practice by Microsoft [...]