Been having weird DNS problems here for some time. Mainly with
Remote SMTP servers replying to our "mail from" with "Domain of sender

address x does not exist" and giving "Host unknown" when trying to
send to
us. Finally got around to tracking them down. The problem is
will respond with a NXDOMAIN to AAAA requests for a domain that has no

AAAA records. The cached NXDOMAIN response will prevent subsequent A

queries succeeding as NXDOMAIN is type independent. The correct
is "NOERROR". This is returned for hosts in the domain that have no
record. This affects mail servers primarily because Sendmail will do
AAAA lookup for inbound messages and only try an A lookup if the AAAA

fails. Once the DNS server has NXDOMAIN in its cache, all future
will fail until the cache times out. BIND 8 was a little less strict
its interpretation of NXDOMAIN, and appears to still work.