mx.post.lu has no CNAME records.
Question and response
Question
dig mx.post.lu. NS
Answer
The response has no answer records.
Authority
post.lu. 282 SOA ns2.pt.lu. hostmaster.ns1.pt.lu. 2026021901 10000 1800 2419200 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0