Apex domains cannot have CNAME records.
Question and response
Question
dig smtp-out01.seaservers.net. NS
Answer
smtp-out01.seaservers.net. 5 NS ns2.hwrn.net.
smtp-out01.seaservers.net. 5 NS ns1.hwrn.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0