Ever since we migrated to a new ISP we have getting alot of mail failing to send to certain domains. Moslty to South African (co.za) domains with the follwing error:
418.104.22.168 Primary target IP responded with: "421 4.2.1 Unable to connect." Attempted failover to alternate host, but that did no succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
We were able to send to these users before but since the migration we cannot.