Hi there,
Without knowing the url of your site, in general, that error can happen when you have a DMARC policy set up for your sender domain with “p=quarantine” or “p=reject” policy and you are using our MailPoet Sending Service.
You can review the information here to see what could be going wrong there: https://kb.mailpoet.com/article/295-spf-dkim-dmarc#troubleshooting
