mail.o04.xemis.net has no CNAME records.
Question and response
Question
dig mail.o04.xemis.net. NS
Answer
The response has no answer records.
Authority
o04.xemis.net. 8844 SOA ns.o04.xemis.net. postmaster.o04.xemis.net. 2025021102 10800 3600 1209600 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0