spf2.amazon.com has no NS records.
NS records for this zone are defined at amazon.com.
NS records for this zone are defined at amazon.com.
Question and response
Question
dig spf2.amazon.com. NS
Answer
The response has no answer records.
Authority
amazon.com. 900 SOA dns-external-master.amazon.com. hostmaster.amazon.com. 2010197829 180 60 604800 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0