mx.citibob.com has no NS records.
NS records for this zone are defined at citibob.com.
NS records for this zone are defined at citibob.com.
Question and response
Question
dig mx.citibob.com. NS
Answer
The response has no answer records.
Authority
citibob.com. 3600 SOA dns.citibob.com. hostmaster.citibob.com. 1376930663 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0