ns4.level3.net has no MX records.
Question and response
Question
dig ns4.level3.net. MX
Answer
The response has no answer records.
Authority
Level3.net. 3600 SOA ns2.Level3.net. dns.level3.net. 2026022207 7200 600 2592000 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0