I’m having the same issues.
@gmail.com addresses work fine, but nothing else.
I’ve tried multiple other domain/mail providers (yahoo.com, InterMedia, my own mail server, - all get 521 5.3.0 Upstream error.
It was working but stopped at some point.
All DNS records are good.
Even tried deleting and re-adding things.
I’ve tried different domains in Cloudflare, same results. It is only working if you use a @gmail.com address as far as I can tell.