sendersv4b.mail.linode.com has no CNAME records.
Question and response
Question
dig sendersv4b.mail.linode.com. NS
Answer
The response has no answer records.
Authority
linode.com. 8662 SOA a1-249.akam.net. dns.linode.com. 2026030634 4200 3600 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0