Non-Delivery Report (NDR)

Document created by user.oxriBaJeN4 Employee on Sep 14, 2015Last modified by user.oxriBaJeN4 Employee on Oct 2, 2015
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A Non-Delivery Report (NDR) or ‘bounce message’ is a report generated by the mail server to inform the sender that the email delivery was not successful.  There are many reasons why a sender will receive an NDR, but typically it is based on a SMTP rejection code, or it would be a timeout failure to deliver an email.


An NDR is typically generated by the senders mail server based on a SMTP 500 error code from the recipient mail server.  Some examples include invalid recipient email address, mailbox full, mail too large, etc..  The recipients mail server would only generate the NDR if an email was accepted and was then undeliverable.


Mimecast generates NDRs for two types of outbound failed delivery types - hard bounced and soft bounced emails.

  • Hard bounced emails:
  • If the email delivery was hard bounced, this means that the recipients mail server rejected the email during the attempted connection.
  • Soft bounced emails:
  • If the email was soft bounced, this is a result of the message being undeliverable based on Mimecast’s retry schedue.


Typically this is due to MX records being unavailable for the recipient’s domain or if Mimecast is unable to connect to the recipients mail server (Host).


How do I use an NDR?

Non-Delivery Reports can be very important and helpful when troubleshooting failed email delivery.  The Administrator should always try and obtain the NDR from their end user when troubleshooting outbound failed email delivery queries.


NDRs contain information such as:


  • Date and time the message was bounced
  • Details of the mail server that bounced it
  • The reason that it was bounced (e.g. invalid user, mailbox full)
  • Details of the bounced message (e.g. sender, recipient, subject)


For more information on bounced emails, view this related article.