ns4.markmonitor.com has no MX records.
Question and response
Question
dig ns4.markmonitor.com. MX
Answer
The response has no answer records.
Authority
markmonitor.com. 300 SOA ns1.markmonitor.com. hostmaster.markmonitor.com. 2026020601 10800 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0