The error message you referenced is an ESMTP error code. Cloudflare does not offer general SMTP services, so you are unlikely to find that response explained here. The message is informing you that the receiving system has reached a permanent error Delivery not authorized, message refused. In this case the receiving system appears to be your email provider. The error suggests that your email provider’s server does not like the Message-ID generated by your GA4 software. You may need to consult with both vendors to find a solution. It doesn’t look like anything that you can fix with your Cloudflare setup.