_spf.btbpo.net has no NS records.
NS records for this zone are defined at btbpo.net.
NS records for this zone are defined at btbpo.net.
Question and response
Question
dig _spf.btbpo.net. NS
Answer
The response has no answer records.
Authority
btbpo.net. 1800 SOA dns1.p08.nsone.net. domains.bottomline.com. 1646648820 3600 600 604800 1800
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0