nspub1.dns.lu has no MX records.
Question and response
Question
dig nspub1.dns.lu. MX
Answer
The response has no answer records.
Authority
dns.lu. 7200 SOA ns1.dns.lu. hostmaster.restena.lu. 1772105464 3600 300 2592000 7200
Additional
. 0 OPT ; payload 1452, xrcode 0, version 0, flags 0