Q. My mail delivery fails with DNS: Service unavailable error. How do I fix this error under Linux / UNIX operating systems?
A. This error indicate that your DNS name resolution is not working correctly. Make sure dns client configuration is correct and your dns servers (or ISPs DNS servers) are working.
Also make sure the remote mail server receiving the mail from the local server is unable to resolve the local mail servers hostname. The local mail server needs to have a valid working fully qualified domain name (FQDN).