Email routing giving "521 5.3.0 Upstream error. TuhicvK1UwOp" error

I set Email Routing on Cloudflare all @aaa.aaa sent to
[email protected]

And I send the mail test in this way:

[email protected] Send to [email protected]

then get [email protected] the error message
521 5.3.0 Upstream error.

In addition, I found that my Email Routing - Email DNS records kept spinning in circles without responding