Understanding SPF, DKIM, and DMARC: A Simple Guide

I would add https://www.mail-tester.com/ to the testing tool list

the free version of https://eu.dmarcian.com (which is great btw, but costs a bit out of my budget) is good, too

this is good too https://www.20i.com/blog/dkim-demystified/

dmarcian won't load in NL:

> On February 20th 2024, the Hague Appellate court instructed dmarcian to block dmarcian.com visitors who originate from Dutch Internet Protocol addresses.

Apparently they had some trademark dispute with their local representative. The story on their website seems to be missing some key details to understand the whole picture.