mx.o2switch.fr has no CNAME records.
Question and response
Question
dig mx.o2switch.fr. NS
Answer
The response has no answer records.
Authority
o2switch.fr. 6123 SOA dark.o2sdns.fr. support.o2switch.fr. 2010032908 86400 7200 1728000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0