ns2.dnssub.com has no MX records.
Question and response
Question
dig ns2.dnssub.com. MX
Answer
The response has no answer records.
Authority
dnssub.com. 60 SOA ns0.dnssub.com. hostmaster.dnssub.com. 2024061902 1800 1800 1209600 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0