Dean;
When you get an NXDOMAIN DNS protocol reply, the DNS protocol (RFC 1034,
etc) defines a specific meaning.
Neither rfc1034 nor rfc1035 define "NXDOMAIN DNS protocol reply.
But when you don't get NXDOMAIN, there is
no meaning to be implied. This is a fact due to the inclusion of wildcard
records in the DNS protocol.
Wrong.
As is clearly stated in rfc1034:
The general
idea is that any name in that zone which is presented to server
in a query will be assumed to exist, with certain properties,
unless explicit evidence exists to the contrary.
domain names matching a wildcard is assumed to exist.
Masataka Ohta