When I lookup your domain thuijls.net on MXToolBox I get the following messageL
Dns record not found
They explain this to mean:
More Information About Dns Record Published
We did not find a SPF (TXT) record for the domain. Either you are using the deprecated type "SPF" record (in which case you will see a warning message for this alongside this message) or no record exists.
For a SPF record to be valid, it must at least contain v=spf1
A TXT lookup for thuijls.net yeilds:
Type Domain Name TTL Record
TXT thuijls.net 5 min v=spf1 include:spf.messagingengine.com ?all
The problem is the ?all - you probably copy-pasted this from somewhere and the (non-standard) hyphen became a question mark.
Edit your TX record to read
v=spf1 include:spf.messagingengine.com -all and all will be good.
Then you can add a 2nd TXT entry for your