mx4.astrum-nival.com has no CNAME records.
Question and response
Question
dig mx4.astrum-nival.com. NS
Answer
The response has no answer records.
Authority
astrum-nival.com. 479 SOA ns1.astrum-nival.com. noc.astrum-nival.com. 2011112901 300 300 2419200 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0