spf-b.authsmtp.com has no CNAME records.
Question and response
Question
dig spf-b.authsmtp.com. NS
Answer
The response has no answer records.
Authority
authsmtp.com. 7178 SOA ns0.dmpriest.net.uk. root.dmpriest.com. 2025120302 3600 1800 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0