Unknown error: permanent error (521): 5.3.0 Upstream error

I apparently stopped receiving email a few months ago (get very little on this particular address) and I am needing some help resolving why. I did not make any changes to any settings, so I am not sure why email just stopped. Sending works fine, it is just receiving:

Unknown error: permanent error (521): 5.3.0 Upstream error, please check Postmaster · Cloudflare Email Routing docs for possible reasons why. Oi8dHH3nG3C2

This one would indicate that you are stacking multiple Email Routing domains on top of each other.

You will need to follow the chain of Email Routing domains and check the “Activity Log”, not only on the first domain, but also the next, and next, and next, until you reach the error from the actual final destination.