spf.mandrillapp.com has no NS records.
NS records for this zone are defined at mandrillapp.com.
NS records for this zone are defined at mandrillapp.com.
Question and response
Question
dig spf.mandrillapp.com. NS
Answer
The response has no answer records.
Authority
mandrillapp.com. 900 SOA ns-720.awsdns-26.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0