You were messing with your cluster on a closed, separate VLAN, but now hundreds of error messages wait in the Postfix queue, ready to ship when your server regain access to the network.
What to do?
You can clean up your Postfix mail queue with postsuper:
# postsuper -d ALL postsuper: Deleted: 4789 messages
As usual, check the man page for more info and the caveats.