_spf.salesforce.com has no CNAME records.
Question and response
Question
dig _spf.salesforce.com. NS
Answer
The response has no answer records.
Authority
salesforce.com. 600 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2016652628 600 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0