Non-Delivery Reports


Non-delivery reports (NDRs) are usually the first indication of a mail system issue that a sender of an e-mail message will receive. There are many different reasons why a message might not be delivered to the recipient. The following table lists the NDR codes with their respective possible cause and troubleshooting recommendations when available.

Common NDR Codes, Possible Cause, and Troubleshooting Information
CodePossible Cause - Troubleshooting


Out-of-memory or out-of-disk space condition on the Exchange server. Potentially also means out-of-file handles on IIS.


Message deleted from a queue by the administrator via the Queue Viewer interface in Exchange System Manager.


Host not responding. Check network connectivity. If problem persists, an NDR will be issued.


Connection dropped. Possible temporary network problems.


Maximum hop count for a message has been exceeded. Check the message address, DNS address, and SMTP virtual servers to make sure that nothing is causing the message to loop.


Message expired. Message wait time in queue exceeds limit, potentially due to remote server being unavailable.


Generic message for no route is available to deliver a message or failure. If it is an outbound SMTP message, make sure that an address space is available and have proper routing groups listed.


Message categorizer failures. Check the destination addresses and resend the message. Forcing rebuild of Recipient Update Service (RUS) may resolve the issue.


Recipient could not be resolved. Check the destination addresses and resend the message. Potentially e-mail account no longer exists on the destination server.


Bad address.


Duplicate SMTP address. Use LDIFDE or script to locate duplicate and update as appropriate.


Local mail system rejected message, “over size” message. Check the recipient’s limits.


Message too large. Potentially the recipient mailbox is disabled due to exceeding mailbox limit.


The remote server has run out of disk space to queue messages, possible SMTP protocol error.


Message loopback detected.


Authoritative host not found. Check message and DNS to ensure proper entry. Potential error in smarthost entry or SMTP name lookup failure.


No route found to next hop. Make sure connectors are configured correctly and address spaces exist for the message type


Categorizer problems with recipient. Recipient may have alternate recipient specified looping back to self.


Looping condition detected. Server trying to forward the message to itself. Check smarthost configuration, FQDN name, DNS host and MX records, and recipient policies.


Generic SMTP protocol error.


SMTP protocol error for receiving out of sequence SMTP protocol command verbs. Possible to low disk space/memory of remote server.


Too many recipients in the message. Reduce number of recipients in message and resend.


Access denied. Sender may not have permission to send message to the recipient. Possible unauthorized SMTP relay attempt from SMTP client.

For further information about troubleshooting NDRs, see part four of the Exchange Server Transport and Routing Guide, available at

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