Below is the collection of known NDR codes, the table is handy to get a quick idea of what caused the NDR
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.
Possible Cause – Troubleshooting
The recipient has exceeded their mailbox limit. It could also be that the delivery directory on the Virtual server has exceeded its limit. (Default 22 MB)
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.Troubleshooting: This code may be caused by transient network issues or servers that are down. The server tries to deliver the message for a specific time period, and then generates additional status reports.
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.
A DNS problem. Check your smart host setting on the SMTP connector. For example, check correct SMTP format. Also, use square brackets in the IP address [126.96.36.199] You can get this same NDR error if you have been deleting routing groups.
Multi-language situation. Your server does not have the correct language code page installed
Problem with email address
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.
Often seen with contacts. Check the recipient address.
Recipient could not be resolved. Check the destination addresses and resend the message. Potentially e-mail account no longer exists on the destination server.
Another problem with the recipient address. Possibly the user was moved to another server in Active Directory. Maybe an Outlook client replied to a message while offline
SMTP; 550 Host unknown. An error is triggered when the host name can't be found. For example, when trying to send an email to bob@ nonexistantdomain.com.
Bad address. Another problem with contacts. Address field maybe empty. Check the address information
Duplicate SMTP address. Use LDIFDE or script to locate duplicate and update as appropriate.
Two objects have the same address, which confuses the categorizer.
Or use Custom search in AD to figured out which object have the same SMTP proxy address
ProxyAddreses=SMTP:firstname.lastname@example.org (change the SMTP proxy address)
See my blog for saved queries very useful, and handy
NDR caused by a problem with the large size of the email.
Local mail system rejected message, "over size" message. Check the recipient's limits.
The message is too large. Else it could be a permissions problem. Check the recipient's mailbox
The recipient has exceeded their mailbox limit.
Message too large. Potentially the recipient mailbox is disabled due to exceeding mailbox limit.
Most likely, a distribution list or group is trying to send an email. Check where the expansion server is situated.
Mail system full. Possibly a Standard edition of Exchange reached the 16 GB limit
System not accepting network messages. Look outside Exchange for a connectivity problem
The remote server has run out of disk space to queue messages, possible SMTP protocol error.
Recipient cannot receive messages this big. Server or connector limit exceeded
Message too big. Check limits, System Policy, connector, virtual server
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 answer from host. Not Exchange's fault check connections
Routing server failure. No available route
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.
Message expired. Message wait time in queue exceeds limit, potentially due to remote server being unavailable
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.
Wrong protocol version
More than 250 attachments
Access denied. Sender may not have permission to send message to the recipient. Possible unauthorized SMTP relay attempt from SMTP client.
Distribution list cannot expand and so is unable to deliver its messages
Check external IP address of ISA server. Make sure it matches the SMTP publishing rule
Extra security features not supported. Check delivery server settings
Cryptographic failure. Try a plain message with encryption
Certificate problem, encryption level maybe to high
Message integrity problem.