_spf.smtp.com has no NS records.
NS records for this zone are defined at smtp.com.
NS records for this zone are defined at smtp.com.
Question and response
Question
dig _spf.smtp.com. NS
Answer
The response has no answer records.
Authority
smtp.com. 900 SOA ns-1304.awsdns-35.org. awsdns-hostmaster.amazon.com. 2 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0