ns4.dn-s.org has no MX records.
Question and response
Question
dig ns4.dn-s.org. MX
Answer
The response has no answer records.
Authority
dn-s.org. 300 SOA nsauth.dn-s.org. hostmaster.dn-s.org. 2026011605 600 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0