_spf.mainloop.net has no NS records.
NS records for this zone are defined at mainloop.net.
NS records for this zone are defined at mainloop.net.
Question and response
Question
dig _spf.mainloop.net. NS
Answer
The response has no answer records.
Authority
mainloop.net. 600 SOA ns1.mainloop.net. hostmaster.mainloop.net. 3959 600 100 604800 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0