mxfallback.spamfilterserver.com has no CNAME records.
Question and response
Question
dig mxfallback.spamfilterserver.com. NS
Answer
The response has no answer records.
Authority
spamfilterserver.com. 3443 SOA ns1.thednscompany.com. ns1.thednscompany.com. 2026012919 28800 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0