_spf.plusserver.com has no CNAME records.
Question and response
Question
dig _spf.plusserver.com. NS
Answer
The response has no answer records.
Authority
plusserver.com. 900 SOA ns1.plusserver.com. hostmaster.plusserver.com. 2026021109 3600 900 3600000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0