Message Routing - Non Delivery Report (NDR)

This article contains information on Non-Delivery Reports (NDRs), their types (hard and soft bounces), causes, and how administrators can use them to troubleshoot failed email deliveries, including key details provided in NDRs for analysis.

A Non-Delivery Report 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 an SMTP rejection code, or it would be a timeout failure to deliver an email.

An NDR is typically generated by the sender's mail server based on an SMTP 500 error code from the recipient's mail server. Some examples include invalid recipient email address, mailbox full, mail too large, etc. The recipient's mail server would only generate the NDR if an email was accepted and was then undeliverable. The receipts for your bounces will be available in your email reports for 30 days after the email is sent.

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 recipient's 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 schedule.

Typically, this is due to MX records being unavailable for the recipient's domain or Mimecast is unable to connect to the recipient's mail server (host).

Emails rejected by Housekeeping from On-Hold queues will not generate NDRS.

Process of Using NDRs

Non-Delivery Reports can be very important and helpful when troubleshooting failed email delivery. The Administrator should always try to 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, see Policies - Mimecast SMTP Error Codes.