mx.security-mail.net has no CNAME records.
Question and response
Question
dig mx.security-mail.net. NS
Answer
The response has no answer records.
Authority
security-mail.net. 5339 SOA ns1.gandi.net. hostmaster.gandi.net. 1771459200 10800 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0