_spf.netwerkplan.nl has no MX records.
Question and response
Question
dig _spf.netwerkplan.nl. MX
Answer
The response has no answer records.
Authority
netwerkplan.nl. 1800 SOA maxim.ns.cloudflare.com. dns.cloudflare.com. 2396894835 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0