ns6.blacknight.com has no MX records.
Question and response
Question
dig ns6.blacknight.com. MX
Answer
The response has no answer records.
Authority
blacknight.com. 3600 SOA ns1.blacknight.com. hostmaster.blacknight.com. 1276183043 14400 7200 2419200 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0