Optonline doesn’t appear to support DMARC (check reporters here: https://us.dmarcian.com/dmarc-data-providers/), so it’s unlikely that the rejection you’re experiencing is due to a DMARC failure.
At the same time, it’s worthwhile to make sure that your messages are actually passing DMARC (simply verifying that you have a DMARC policy, that you have a DKIM key, that you have SPF in place, is not enough) by using a tool such as dmarcian’s RUA analysis service to see whether reported traffic is actually passing DMARC successfully.
If that all checks out, you might have to continue trying to get a response out of optonline.
