A non-delivery report (NDR) is an automated email message from the mail system informing the sender that there was a delivery problem with an email message that they sent.
Email Gateway Defense (EGD) does not allow outbound NDRs through our systems as it is considered to be a form of spam called backscatter. Backscatter emails are improper automated bounce messages sent by your mail server and can lead to having the Barracuda Networks IP ranges placed onto blocklists.
To minimize the risk of sending backscatter, considering the following:
- Invalid recipients – Ensure you are doing proper recipient verification. This can be done on your mail server or through the Email Gateway Defense. For more information, see Configuring Recipient Verification.
- Message too large – The Email Gateway Defense has a 300MB default size limit. If your mail server is configured to accept less than 300MB, contact Barracuda Networks Technical Support to have your EGD limit lowered to match your mail server.
- Local policies – Ensure that any local policies you have configured on your mail server are migrated to the Email Gateway Defense.