This error occurs when the DNS records (that convert hostnames to IP addresses for actual use) are altered or non-existant (no authoritative DNS server could be located).
* The DNS server for that hostname is down.
* The DNS records for that domain are no longer active or valid.
* The new DNS records for a domain which has changed IPs have not yet propagated so they still point to the old IP address, which can also give a Connection Refused error or Connection Timed Out error.