I’ve moved my domain over some while ago to deSEC.
I am very sure my SPF record is also in my smartict.nl domain and correctly formatted.
Nevertheless mxtoolbox and other sites tell me there is no SPF record at all.
What am I doing wrong here?
Well, there is no TXT record for the domain smartict.nl:
$ dig +short smartict.nl txt
$
So from the outside there is no way of telling what you did wrong. Maybe post a screenshot of the web UI where the data you entered as a TXT record is shown?
You need to leave the Subname field blank (or, if the UI allows, set it to smartict.nl. with a period at the end). You have created the subdomain smartict.nl for your domain smartict.nl and set the TXT record there.
Sorry, I just tried this and that does not not work in the deSEC web UI. Neither is @ a valid Subname. So ignore that part of my comment and just leave the Subname field empty.