I have a number of messages backing up in an exchange queue.
I have ran the MS Exchange Troubleshooting Assistant which has identified issues but hasn't helped much with the resolution.
The 'Remote delivery' queue (hoex2\39502) on server hoex2 is in retry status. Number of message(s) in the queue: 598. Last error information: 451 4.4.0 Primary target IP address responded with: "421 4.7.0 Too many errors on this connection, closing transmission channel." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.