Domain {1} is a remote domain to which server {2} is trying to send messages but neither the 'MX' records nor 'Host' records ...

Domain {1} is a remote domain to which server {2} is trying to send messages but neither the 'MX' records nor 'Host' records of domain {1} can be obtained from any DNS server {2} uses. This may be causing message backups in the queue or non-delivery reports if the DNS response is 'non-existent domain'.