Apex domains cannot have CNAME records.
Question and response
Question
dig mail.tibus.net. NS
Answer
mail.tibus.net. 1 NS dns2.tibus.net.
mail.tibus.net. 1 NS dns1.tibus.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0