_spf6.yourfilter.nl has no MX records.
Question and response
Question
dig _spf6.yourfilter.nl. MX
Answer
The response has no answer records.
Authority
yourfilter.nl. 300 SOA ns111.auroradns.eu. admin.auroradns.eu. 2019012201 86400 7200 604800 300
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0