Apex domains cannot have CNAME records.
Question and response
Question
dig mx25.net. NS
Answer
mx25.net. 5 NS dns3.p01.nsone.net.
mx25.net. 5 NS dns1.p01.nsone.net.
mx25.net. 5 NS dns2.p01.nsone.net.
mx25.net. 5 NS dns4.p01.nsone.net.
Authority
The response has no authority records.
Additional
dns1.p01.nsone.net. 1 A 198.51.44.1
dns2.p01.nsone.net. 1 A 198.51.45.1
dns3.p01.nsone.net. 1 A 198.51.44.65
dns4.p01.nsone.net. 1 A 198.51.45.65
dns1.p01.nsone.net. 1 AAAA 2620:4d:4000:6259:7:1:0:1
dns2.p01.nsone.net. 1 AAAA 2a00:edc0:6259:7:1:0:0:2
dns3.p01.nsone.net. 1 AAAA 2620:4d:4000:6259:7:1:0:3
dns4.p01.nsone.net. 1 AAAA 2a00:edc0:6259:7:1:0:0:4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0