mx.shallsbarcelona.com has no NS records.
NS records for this zone are defined at shallsbarcelona.com.
NS records for this zone are defined at shallsbarcelona.com.
Question and response
Question
dig mx.shallsbarcelona.com. NS
Answer
The response has no answer records.
Authority
shallsbarcelona.com. 3600 SOA dns.shallsbarcelona.com. hostmaster.shallsbarcelona.com. 1252873252 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0