mx2.mail.level3.net has no CNAME records.
Question and response
Question
dig mx2.mail.level3.net. NS
Answer
The response has no answer records.
Authority
Level3.net. 2455 SOA ns2.Level3.net. dns.level3.net. 2026022217 7200 600 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0