mx.ptmail.sapo.pt has no CNAME records.
Question and response
Question
dig mx.ptmail.sapo.pt. NS
Answer
The response has no answer records.
Authority
ptmail.sapo.pt. 3600 SOA dns01.sapo.pt. root.ns.sapo.pt. 1771953947 800 200 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0