I was unable to find where to actually create a ticket (are free users even able to?), but I did stumble upon the “Error messaging” tool and entered “521” as the error code. This led me to a similar community post ( Send email and get 521 upstream error ) 5.3.0 Upstream error. zY5Aud4kKVqy (in reply to RCPT TO command)which I hadn’t found earlier. The accepted answer on that post stated the problem is due to an infinite loop in the configuration.
While I’m not sure how this can be (I can send email directly to {redacted} with no issue at all), I changed the action “Send to” address to be the actual gmail account that the message would ultimately end up on, and I am now able to send without issue.