Non-Delivery Report / Receipt (NDR)

A Non-Delivery Report/Receipt (NDR), or simply a bounce, is an automated electronic mail message from a mail system informing the sender of another message about a delivery problem. The original message is said to have bounced. Errors can occur at multiple places in mail delivery. A sender may sometimes receive a bounce message from the sender's mail server, and other times from a recipient's mail server. With the rise in forged spam and e-mail viruses, users now frequently receive erroneous bounce messages sent in response to messages they never actually sent.

There are two types of NDRs: transient and permanent. A transient NDR is generated when the server couldn't temporarily deliver the message. If delivery continues to fail after multiple attempts, a permanent failure NDR is generated.

Why Did I Receive an NDR?

There are many reasons why an e-mail may bounce. One reason is if the recipient address is misspelled, or simply does not exist on the receiving system. This is a user unknown condition. Other reasons include resource exhaustion - such as a full disk - or the rejection of the message due to spam filters.

Typically, a bounce message will contain several pieces of information to help the original sender in understanding the reason the message was not delivered:

  • The date and time the message was bounced,
  • The identity of the mail server that bounced it,
  • The reason that it was bounced (e.g. user unknown or mailbox full),
  • The headers of the bounced message, and
  • Some or all of the content of the bounced message.

Enhanced Mail System Status Codes (RFC 3463), as defined by The Internet Engineering Task Force (IETF), describes the codes used to indicate the bounce reason. Common codes are 5.1.1 (Unknown user), 5.2.2 (Mailbox full), and 5.7.1 (Rejected by security policy/mail filter).

Backscatter

Backscatter occurs when spam or viruses send mail using a forged email address as the return path. This can lead to thousands of bogus delivery status notices (DSN), vacation and out-of-office messages, auto-responders, etc., ending up in the inbox.

Backscatter Protection, a feature of MDaemon Messaging Server and SecurityGateway for Email Servers for Exchange/SMTP Servers, distinguishes between legitimate and unauthorized use of your email address in the MAIL FROM: return path. By protecting the return path, MDaemon and SecurityGateway can determine whether a certain class of messages, such as DSNs, vacation notices, and auto-responders are valid or not.