SocketError: WSAHOST_NOT_FOUND No such host is known.

I'm getting this error when trying to send email:

smtpSocketConnect:
socket2Connect:
  connect2:
    connectSocket:
      connect_ipv6_or_ipv4:
        resolveHostname6:
          getAddressInfo:
            Failed to get host address info. (3)
            SocketError: WSAHOST_NOT_FOUND No such host is known.
            hostOrIpAddr: smtp.mail.something.com.au
            port: 25
            Versions of Windows earlier than Windows XP are limited to handling IPv4 only
            On Windows Server 2003 and Windows XP, IPv6 addresses are returned only if IPv6 is installed on the local computer.
          --getAddressInfo
        --resolveHostname6
        Domain to IP address resolution failed.
      --connect_ipv6_or_ipv4
    --connectSocket
    ConnectFailReason: DNS lookup failed
  --connect2
--socket2Connect
Failed to connect to SMTP server..
--smtpSocketConnect

What could be the problem?