_spf.plan.io has no CNAME records.
Question and response
Question
dig _spf.plan.io. NS
Answer
The response has no answer records.
Authority
plan.io. 2509 SOA ns1.jonaspasche.com. hostmaster.plan.io. 1764781292 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0