When you run dcdiag /test:dns on Windows Server 2003 SP1 (Service Pack 1), you receive an error like:
Error: Forwarders list has invalid forwarder: < IP Address of DNS server> (<name unavailable>) Name resolution is not functional. _ldap._tcp.test.com failed on the DNS server < IP Address of DNS server>
This behavior occurs when you have configured conditional forwarding on the DNS server AND you run the dcdiag /test:dns command, because it queries all the conditional forwarders for the DNS server.
NOTE: This error message is harmless and may safely be ignored.