_spfb.cloud.sg has no NS records.
NS records for this zone are defined at cloud.sg.
NS records for this zone are defined at cloud.sg.
Question and response
Question
dig _spfb.cloud.sg. NS
Answer
The response has no answer records.
Authority
cloud.sg. 300 SOA ns-cloud-c1.googledomains.com. cloud-dns-hostmaster.google.com. 3 21600 3600 259200 300
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0