_spf.topdns.com has no NS records.
NS records for this zone are defined at topdns.com.
NS records for this zone are defined at topdns.com.
Question and response
Question
dig _spf.topdns.com. NS
Answer
The response has no answer records.
Authority
topdns.com. 3600 SOA ns-uk.topdns.com. hostmaster.topdns.com. 2026021803 43200 900 1209600 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0