In SMTP, receiving a 5xx error is considered permanent.

Historically this is the WORST thing that can happen to an email message.

The remote server is telling the sending server to delete the message and to no longer queue. 

If your server responds with 5xx code the mail server doesn't queue the message and retry the delivery later, it drops it. 

A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made before a successful delivery. 

Typical Use Case:

  1. There are times that your on-premise mail server explodes causing downtime and needs to be reinstalled from scratch. 

  2. You decide to rebuild the machine or reinstall it.
  3. You do that but do not block the firewall, so as soon as the server comes back online we will start pushing the queued mail to it.

  4. The server responds to the SMTP requests before you have had a chance to define your users or to adjust your settings. 
  5. But most mail server software is configured by default to reject emails with a 5xx error for mailboxes that don't exist. The admin has not yet set up any accounts on the brand-new server so all mail is rejected and lost.
  6. Message replay keeps even 5xx rejected mail queued on the DuoCircle mail server allowing for those messages to be reset again and again during the retention perior even if they were rejected before - thus Message Replay