mx1.mail.level3.net has no NS records.
NS records for this zone are defined at level3.net.
NS records for this zone are defined at level3.net.
Question and response
Question
dig mx1.mail.level3.net. NS
Answer
The response has no answer records.
Authority
Level3.net. 3600 SOA ns2.Level3.net. dns.level3.net. 2026022208 7200 600 2592000 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0