We ran into this as well. The cause of the infinite loop is because we are sending an “Auto Generated Reply” back to email addresses that we detect as “no-reply” or “auto-generated”. If pop3_polling_username is similar to notification_email, the email that bounced will be polled for and we end up starting another cycle.
It does seem really weird to me that we reply to email addresses that we detect as “no-reply” so I opened a PR to stop doing that.