Oh well, I should have read a little more carefully.
The full header would help to see where the problem could be located...
However, from what I see, it is this message:
550, "5.7.1", Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked. For more information, review this article.
The article: https://support.google.com/mail/answer/188131
You might want to check it, as it has recommendations to avoid this.
I'm pretty sure it's not because of the missing DNS record.
DMARC is not really required but optional.