I've read through the documentation available on this issue but just haven't managed to set up all up properly.
(Like: https://help.directadmin.com/item.php?id=569)

SPF is validated correctly.

Part of the problem i think is: the website sends email from: header.from=aaabbb.nl, but [email protected] (vps domain name). So i generated a DKIM for the domain aaabbb.nl but this isn't detected / implemented correctly i think. For example, what should the VPS dkim "Selector" be?

Original message content:
spf=pass (google.com: domain of [email protected] designates 111.111.111.111 as permitted sender) [email protected];
dmarc=fail (p=QUARANTINE sp=NONE dis=NONE) header.from=aaabbb.nl

Some help would be highly appreciated!