mta-b.6level.net has no MX records.
Question and response
Question
dig mta-b.6level.net. MX
Answer
The response has no answer records.
Authority
6level.net. 300 SOA ns1.6level.net. admin.6level.net. 2026022300 3600 300 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0