firewall.esker.fr has no MX records.
Question and response
Question
dig firewall.esker.fr. MX
Answer
The response has no answer records.
Authority
esker.fr. 3600 SOA ns.esker.fr. dnsadm.esker.fr. 2025111013 1800 900 604800 3600
Additional
. 0 OPT ; payload 4000, xrcode 0, version 0, flags 0