mx.sapo.pt has no NS records.
NS records for this zone are defined at sapo.pt.
NS records for this zone are defined at sapo.pt.
Question and response
Question
dig mx.sapo.pt. NS
Answer
The response has no answer records.
Authority
sapo.pt. 3600 SOA ns.sapo.pt. root.sapo.pt. 1771953947 3600 7200 2419200 300
Additional
The response has no additional records.