customer.Level3.net has no MX records.
Question and response
Question
dig customer.Level3.net. MX
Answer
The response has no answer records.
Authority
customer.level3.net. 3600 SOA ns1.Level3.net. dns.level3.net. 2019010900 7200 600 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0