smtp.restena.lu has no MX records.
Question and response
Question
dig smtp.restena.lu. MX
Answer
The response has no answer records.
Authority
restena.lu. 3600 SOA ns1.restena.lu. noc.restena.lu. 1771882445 14400 1800 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0