spf1.markmonitor.com has no CNAME records.
Question and response
Question
dig spf1.markmonitor.com. NS
Answer
The response has no answer records.
Authority
markmonitor.com. 294 SOA ns1.markmonitor.com. hostmaster.markmonitor.com. 2026020601 10800 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0