Apex domains cannot have CNAME records.
Question and response
Question
dig ports.email. NS
Answer
ports.email. 5 NS dns03.ports.se.
ports.email. 5 NS dns04.ports.net.
ports.email. 5 NS dns01.dipcon.com.
ports.email. 5 NS dns02.ports.se.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0