mail.nos.pt has no CNAME records.
Question and response
Question
dig mail.nos.pt. NS
Answer
The response has no answer records.
Authority
nos.pt. 1113 SOA ns5.nos.pt. dns.cloudflare.com. 2398114661 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0