Notification - Non-Delivery Report (NDR)

A non-delivery report (NDR) is a report that is automatically generated by a mail server to inform the sender that their email message was not successfully delivered. The NDR is received in the form of an email from the sender's mail server and contains information on why the delivery was unsuccessful. Most often, this is due to an incorrect destination email address entered into the "To" field or the recipient's inbox being full and no longer able to receive new messages.

Non-delivery reports are also known as non-delivery receipts, bounced messages, non-delivery notifications (NDN) or delivery status notifications (DSN).

Non-delivery reports are generated by mail servers based on a standard SMTP 500 error coding system. Specifically, they are generated by the sender's mail server based on an SMTP 500 error code that is provided by the recipient's mail server. All of these scenarios are valid delivery failure scenarios and have their own SMTP 500 code that informs the sender why the email has bounced.

However, errors that bar the message from being delivered can occur at many stages during mail delivery, such as:

• The sender's mail server being unable to find the receiver mail server for any reason.

• The mail having been received but the server storage suddenly becoming full, resulting in the mailer daemon being unable to replicate the mail.

• The recipient has configured the email server to forward emails to another mail server which has failed for some reason.

All of these scenarios are valid delivery failure scenarios and have their own SMTP 500 codes that inform the sender why the email has bounced.

Sample error codes for NDRs include:

• 4.2.2 — The recipient has exceeded their mailbox limit

• 5.1.x — Problem with the recipient email address

• 5.4.0—DNS problem