We have noticed a situation where Windchill is unable to handle 421 errors from theSMTP server. Basically, when the SMTP server gets too busy, it sends a 421 message back to Windchill. Windchill doesn't know how to handle the error message, and subsequently deletes the message from the email queue. So the end user never receives the email notification.
I am proposing a new solution, which is to have Windchill set the status from Ready to Failed in the email queue in the event that Windchill receives the 421 error from the SMTP server. That way, the Administrator has more control over re-queuing the message to be sent for transmission.