What is the name of the domain?
What is the issue you’re encountering
Remote server returned ‘521 5.3.0 Upstream error, please check Postmaster · Cloudflare Email Routing docs for possible reasons why. 8i0RUOmUHbcc’
Welcome to the Cloudflare Community. 
The ESMTP reply thay you shared is generated by the recipient server operated by your mailbox provider. You will need to consult with your mailbox provider to identify the reason for their response.
Per RFC 7504, the SMTP reply code 521 means that “Host does not accept mail”