smtp20.cnrs.fr has no MX records.
Question and response
Question
dig smtp20.cnrs.fr. MX
Answer
The response has no answer records.
Authority
cnrs.fr. 300 SOA ns0.cnrs.fr. dnsmaster.dsi.cnrs.fr. 2026030602 21600 3600 3600000 300
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0