Understanding everything about NDR

Non Delivery Report

The mail server generates a Non Delivery Report (NDR) or ‘bounce message’ to alert the sender that the email delivery was unsuccessful. A sender may receive an NDR for a variety of reasons, but the most common are an SMTP rejection code or a timeout failure to transmit an email.

An NDR is often generated by the sender’s mail server in response to the recipient’s SMTP 500 error code. Invalid recipient email address, inbox full, letter too big, and so on are some instances. The NDR is only generated by the recipient’s mail server if an email is received and subsequently returned as undeliverable.

What is an NDR?

A non-delivery report (NDR) is a report that a mail server generates automatically to alert the sender that their email message was not delivered properly. The NDR is sent as an email from the sender’s mail server and contains information about why the delivery failed. The most common causes are an incorrect destination email address in the “To” field or the receiver’s inbox being full and unable to receive new messages. Non-delivery receipts, bounced messages, non-delivery notifications (NDN), and delivery status alerts are all terms for non-delivery reports (DSN).

How is an NDR generated?

Mail servers create non-delivery notifications based on the standard SMTP 500 error code scheme. It is generated by the sender’s mail server in response to the receiver’s mail server returning an SMTP 500 error code.¬†

Codes for understanding the status

  • Delivery Not Authorized (NDR 5.7.1)

The words “Delivery Not Authorized” may accompany the 5.7.1 status code. This error usually arises when the sender tries to send a message to a distribution group that they are not allowed to transmit to. Distribution groups are frequently set up such that only remembers are permitted to transmit messages to the group. This error can also occur if the message being sent meets the criteria set out in a transport rule that prevents the message from being delivered.

  • Unable to Relay NDR 5.7.1

The nondelivery report may be another form of the 5.7.1 status code, indicating the exchange was unable to send the message. Generally speaking, this error message implies that the sender attempted to send a message to its intended recipient via an anonymous mail server.

  • 5.2.2 The recipient’s inbox is full, as shown by the status code. This happens when a mailbox has a quota set by an administrator, and the quota has been achieved or exceeded. You won’t be able to send new messages to the recipient until some of the old ones have been deleted from the inbox.
  • 5.3.4 The status code indicates that the message transmitted exceeds the recipient’s mailbox’s size limit. When a message is sent with an extremely big attachment, this sort of non-delivery report is often triggered.
  • 5.4.6 A message loop has occurred, as indicated by this status code. Consider the case when you sent someone a message and they responded with an out of office message.
  • 5.5.3 This status code indicates that there are too many receivers for the message. This occurs when the To, CC, and BCC fields of an email have an excessive number of recipients.

Conclusion 

However, errors that prevent the message from being delivered can occur at any point during the delivery process, such as the sender’s mail server being unable to locate the receiver’s mail server for any reason, the mail being received but the server storage suddenly becoming full, preventing the mailer daemon from replicating the mail, or the receiver configuring the email server to forward emails to another mail server that has failed for some reason. All of these are genuine delivery failure circumstances, and each has its own SMTP 500 code that tells the sender why the email bounced.

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By Michael Caine

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