Are you receiving a bounce-back (NDR - Non-Delivery Report) when sending an email message?  We have provided a list below with basic knowledge on what the codes actually mean. 


4.2.2 - The recipient has exceeded their mailbox limit. It could also be that the delivery directory on the Virtual Server has exceeded it's limit (Default 22 MB).


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


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


4.4.1 - Host not responding.  Check network connectivity.  If problem persists, an NDR will be issues.


4.4.2 - 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.


4.4.6 - 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. 


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


4.4.9 - 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 []  You can get this same NDR error if you have been deleting routing groups.


4.6.5 - Multi-language situation.  Your server does not have t he correct language code page installed.


5.1.X - Problem with email address.


5.0.0 - 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.


5.1.0 - Message categorizer failures.  Check the destination addresses and resend the message.  Forcing rebuild of Recipient Update Server (RUS) may resolve the issue.  Often seen with contacts.  Check the recipient address. 


5.1.1 - 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.


5.1.2 - 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


5.1.3 - Bad address.  Another problem with contacts.  Address field maybe empty.  Check the address information.


5.1.4 - 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. (change the SMTP proxy address).  


5.2.X - NDR caused by a problem with the large size or the email.


5.2.1 - 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.


5.2.2 - The recipient has exceeded their mailbox limit.


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


5.2.4 - Most likely, a distribution list or group is trying to send an email.  Check where the expansion server is situated. 


5.3.1 - Mail system full.  Possibly a Standard edition of Exchange reached the 16GB limit.


5.3.2 - System not accepting network messages.  Look outside Exchange for a connectivity problem.


5.3.3 - 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. 


5.3.4 - Message too big.  Check limits, System Policy, connector, virtual server.


5.3.5 - Message loopback detected.  Multiple Virtual Servers are using the same IP address and port.  


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


5.4.1 - No answer from host.  Not Exchange's fault check connections.


5.4.2 - Bad connection.


5.4.3 - Routing server failure.  No available route.


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


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


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


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


5.5.0 - Generic SMTP protocol error.


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


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


5.5.5 - Wrong protocol version.


5.6.3 - More than 250 attachments.


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


5.7.2 - Distribution list cannot expand and so it is unable to deliver it's messages.


5.7.3 - Check external IP address of ISA server.  Make sure it matches the SMTP publishing rule.


5.7.4 - Extra security features not supported.  Check delivery server settings.


5.7.5 - Cryptographic failure.  Try a plain message with encryption.


5.7.6 - Certificate problem, encryption level maybe too high.


5.7.7 - Message integrity problem.