spf.smtp2go.com has no NS records.
NS records for this zone are defined at smtp2go.com.
NS records for this zone are defined at smtp2go.com.
Question and response
Question
dig spf.smtp2go.com. NS
Answer
The response has no answer records.
Authority
smtp2go.com. 600 SOA ns2.smtpcorp.com. hostmaster.smtp2go.com. 2015074660 14400 3600 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0