mx1.mailbox.pt has no CNAME records.
Question and response
Question
dig mx1.mailbox.pt. NS
Answer
The response has no answer records.
Authority
mailbox.pt. 3428 SOA dns1.host-redirect.com. hostmaster.mailbox.pt. 2026021801 3600 600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0