Email Routing 421Upstream error, bounced emails

Hey all

Custom email is routing through cloudfront to Gmail. Someone just alerted me of bounced emails with a response returning

“421 4.3.0 Upstream error, please check postmaster for possible reasons why. 7bm9wC5Dq5QT”

Email address is verified. I sent a test email from a different gmail account and it seems to be working fine. Any ideas how to resolved this? I’m not finding relevant 421 issues.

Thanks in advance!