Email Routing - one address is rejecting mail (521 5.3.0 Upstream error.)


I am setting up mail routing for our domain, and everything works fine to Gmail addresses - however there is one address (domain is where mail sent from multiple accounts is just getting rejected with, for example:
Remote Server returned ‘521 5.3.0 Upstream error. 73rulR7AiRH1’

The messages don’t show up in the log so I can’t really troubleshoot effectively. I’m guessing it’s a spam rejection, but unfortunately… it’s not spam.

Could support take a look and advise? Or has anyone else been running into this?