The dns query that is being sent is an IPv6 type=aaaa. Meaning they want to know the IPv6 IP address of the domain of the sender. Our dns is not setup to resolve IPv6 queries so it sends NXDOMAIN, or NonExistantDomain. And the message get bounced by their antispam software saying "Domain does not exist"
I am still not sure if IPv4 query is being sent. Or if we are wrong(Non RFC Compliant) by not being capable of resolving IPv6 type=aaaa queries.
Technically, the NXDOMAIN response (from an authoritative nameserver) indicates that the domain in question does not exist. So if they query for the AAAA record for example.com, and an authoritative nameserver for example.com returns NXDOMAIN, it's saying "the example.com domain does not exist."
So lookups for the AAAA record (or A record) for "nonexistenthost.example.com" should return NXDOMAIN, but lookups for the AAAA record (or A record) for "www.example.com" should NOT return NXDOMAIN (unless you don't have a www.example.com).
It sounds like a new anti-spam program is doing this, as we've recently had requests for http://www.DNSreport.com to check for this.
-Scott
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