The error 521 5.3.0 Upstream error, please check Postmaster · Cloudflare Email Routing docs for possible reasons why. m7Gi7ya1Yc4x indicates that there’s an issue with delivering the email to the specified destination server. This is generally due to issue at the upstream mail server.
To address this concern, please reach out to destination mail server for further assistance. They will have necessary logs or relevant tools to further checking on the error delivery.
Additionally, I’ve observed that there’s an error Destination address not found in status here: https://dash.cloudflare.com/63acf18caa8cb688aad3786f33c1fc65/makemehappy.org.uk/email/routing/routes. Please ensure the destination address is correct to prevent any delivery issue.