Apex domains cannot have CNAME records.
Question and response
Question
dig smtpout.spamfiltering.com. NS
Answer
smtpout.spamfiltering.com. 1 NS dns1.xssl.net.
smtpout.spamfiltering.com. 1 NS dns3.xssl.net.
smtpout.spamfiltering.com. 1 NS dns2.xssl.net.
Authority
The response has no authority records.
Additional
dns1.xssl.net. 4 A 84.22.166.196
dns2.xssl.net. 4 A 217.194.210.107
dns3.xssl.net. 4 A 109.169.17.173
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0