Email routing 521 5.3.0 Upstream error

I am trying to set up email routing for my domain and I get the above error when sending test emails. The log show the following:

I don’t understand how the sender is being verified. It looks to me that the destination <> is trying to verify the sender [email protected] at the site that is doing the routing <>

If someone can help me untangle what is happening and how to resolve it, I would appreciate it.

FYI, all three domains are managed by Cloudflare.

Turn off the sender verification in your destination mailserver for any forwarded recipient domains.

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