relay.mail.level3.net has no CNAME records.
Question and response
Question
dig relay.mail.level3.net. NS
Answer
The response has no answer records.
Authority
Level3.net. 3600 SOA ns2.Level3.net. dns.level3.net. 2026022503 7200 600 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0