If we are filtering emails for a customer and the mail server is unreachable and as such a temp 400 SMTP error is generated, we will as per default spool inbound emails for 48 hours and attempt to redeliver emails continually every minute for the first 3 attempts, thereafter for the next 24 hours, we will attempt delivery every ten minutes, the second 24 hour period we will attempt redelivery every 45 minutes on average.

first 24 hours = every 10 minutes on average

second 24 hour period = every 45 minutes on average

If a message is delayed for delivery due to the recipient's server not being available or any other reason, our system generates a DSN (delivery status notification) after 4 hours: This is the exact format.

Delivery is delayed to these recipients or groups:


Subject: anything

This message hasn't been delivered yet. Delivery will continue to be attempted.

The server will keep trying to deliver this message for the next 1 days, 19 hours and 59 minutes. You'll be notified if the message can't be delivered by that time.

Our system will then generate an additional DSN once the full 48 hour period has elapsed. This is the exact format.

Delivery has failed to these recipients or groups:

The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk.

Diagnostic information for administrators:

Generating server: the recipients 

#550 4.4.7 QUEUE.Expired; message expired ##