spf.salesforce.com has no NS records.
NS records for this zone are defined at salesforce.com.
NS records for this zone are defined at salesforce.com.
Question and response
Question
dig spf.salesforce.com. NS
Answer
The response has no answer records.
Authority
salesforce.com. 1800 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2016652926 600 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0