mx1.mail.level3.net has no CNAME records.
Question and response
Question
dig mx1.mail.level3.net. NS
Answer
The response has no answer records.
Authority
Level3.net. 3055 SOA ns2.Level3.net. dns.level3.net. 2026022202 7200 600 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0