_spf.solarwinds.com has no NS records.
NS records for this zone are defined at solarwinds.com.
NS records for this zone are defined at solarwinds.com.
Question and response
Question
dig _spf.solarwinds.com. NS
Answer
The response has no answer records.
Authority
solarwinds.com. 600 SOA a1-139.akam.net. hostmaster.solarwinds.com. 2013093108 1200 600 1209600 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0