mta-31ff35.us.email.fireeyecloud.com has no CNAME records.
Question and response
Question
dig mta-31ff35.us.email.fireeyecloud.com. NS
Answer
The response has no answer records.
Authority
fireeyecloud.com. 1537 SOA bonnie.ns.cloudflare.com. dns.cloudflare.com. 2397141198 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0