Apex domains cannot have CNAME records.
Question and response
Question
dig zix.com. NS
Answer
zix.com. 5 NS ns01.zixcorp.com.
zix.com. 5 NS ns02.zixcorp.com.
zix.com. 5 NS ns03.zixcorp.com.
Authority
The response has no authority records.
Additional
ns01.zixcorp.com. 5 A 63.71.8.12
ns02.zixcorp.com. 5 A 63.71.9.12
ns03.zixcorp.com. 5 A 74.203.184.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0