I've set up a working cron according to the installation instructions. It fires every 5 minutes, but intermittently a user's reply to a ticket email will get stuck in the alert email's inbox causing these system errors:
The cron and email responses work 98% of the time, so I have no idea what causes an email to get stuck. It's happened to several emails on different domains, but I haven't been able to replicate the issue through stress testing. The only thing I can think to do is reduce the cron's interval, but is there any known issue that causes this?