521 5.3.0 Upstream error

Hello Cloudflare-Community,
i have a problem with Cloudflare E-Mail (Beta).
i have the same Error: Email routing giving "521 5.3.0 Upstream error"

Another domains working without any problems.

All another Domains: Any E-Mail Account/Provider —to—> Any [email protected] —forward—> [email protected] = Working/Received

example.io : Any E-Mail Account —to—> Any [email protected] —forward—> [email protected] = not working/not receiving

BUT if i change [email protected] to [email protected] or [email protected], it’s working

example.io : Any E-Mail Account —to—> Any [email protected] —forward—> [email protected]/ [email protected] = Working/Received

Only one domain is not working with a specific adress [email protected]

Any idea, why this domain with the same settings/szenario and address is not working?
I can send my domain via PM, if u want.

SMTP error from remote server for TEXT command, host: amir.mx.cloudflare.net ( reason: 521 5.3.0 Upstream error. fr3pl98HNKBx

Thank you

Can someone help me?