Apex domains cannot have CNAME records.
Question and response
Question
dig mailbox.pt. NS
Answer
mailbox.pt. 3 NS dns4.host-redirect.com.
mailbox.pt. 3 NS dns3.host-redirect.com.
mailbox.pt. 3 NS dns1.host-redirect.com.
mailbox.pt. 3 NS dns2.host-redirect.com.
Authority
The response has no authority records.
Additional
dns1.host-redirect.com. 3 A 91.198.47.1
dns2.host-redirect.com. 3 A 194.8.30.2
dns3.host-redirect.com. 3 A 94.126.175.203
dns4.host-redirect.com. 3 A 185.15.23.202
dns1.host-redirect.com. 3 AAAA 2001:4cc0:4:1:0:0:0:5
dns2.host-redirect.com. 3 AAAA 2001:4cc0:4:10:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0