Coz I always forget them!

http://www.fots.nl/index.php/ndr-and-smtp-reply-and-error-codes/

• 2.0.0 (nonstandard success response, see rfc876)
• 2.1.1 System status, or system help reply
• 2.1.4 Help message
• 2.2.0 Service ready
• 2.2.1 Service closing transmission channel
• 2.5.0 Requested mail action okay, completed
• 2.5.1 User not local; will forward to
• 3.5.4 Start mail input; end with .
• 4.2.1 Service not available, closing transmission channel
• 4.2.2 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)
• 4.3.1 Not enough disk space on the delivery server. Microsoft say this NDR maybe reported as out-of-memory error.
• 4.3.2 Classic temporary problem, the Administrator has frozen the queue.
• 4.4.1 Intermittent network connection. The server has not yet responded. Classic temporary problem. If it persists, you will also a 5.4.x status code error.
• 4.4.2 The server started to deliver the message but then the connection was broken.
• 4.4.6 Too many hops. Most likely, the message is looping.
• 4.4.7 Problem with a timeout. Check receiving server connectors.
• 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 [197.89.1.4] You can get this same NDR error if you have been deleting routing groups.
• 4.5.0 Requested mail action not taken: mailbox unavailable
• 4.5.1 Requested action aborted: local error in processing
• 4.5.2 Requested action not taken: insufficient system storage
• 4.6.5 Multi-language situation. Your server does not have the correct language code page installed.
• 5.0.0 Syntax error, command unrecognised. You get this NDR when you make a typing mistake when you manually try to send email via telnet. More likely, a routing group error, no routing connector, or no suitable address space in the connector. (Try adding * in the address space)
  This status code is a general error message in Exchange 2000. In fact Microsoft introduced a service pack to make sure now get a more specific code.
• 5.0.1 Syntax error in parameters or arguments
• 5.0.2 Command not implemented
• 5.0.3 Bad sequence of commands
• 5.0.4 Command parameter not implemented
• 5.1.x Problem with email address.
• 5.1.0 Often seen with contacts. Check the recipient address.
• 5.1.1 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.
• 5.1.3 Another problem with contacts. Address field maybe empty. Check the address information.
• 5.1.4 Two objects have the same address, which confuses the categorizer.
• 5.1.5 Destination mailbox address invalid.
• 5.1.6 Problem with homeMDB or msExchHomeServerName - check how many users are affected. Sometimes running RUS (Recipient Update Service) cures this problem. Mailbox may have moved.
• 5.1.7 Problem with senders mail attribute, check properties sheet in ADUC.
• 5.2.x NDR caused by a problem with the large size of the email.
• 5.2.1 The message is too large. Else it could be a permissions problem. Check the recipient’s mailbox.
• 5.2.2 Sadly, the recipient has exceeded their mailbox limit.
• 5.2.3 Recipient cannot receive messages this big. Server or connector limit exceeded.
• 5.2.4 Most likely, a distribution list or group is trying to send an email. Check where the expansion server is situated.
• 5.3.0 Problem with MTA, maybe someone has been editing the registry to disable the MTA / Store driver.
• 5.3.1 Mail system full. Possibly a Standard edition of Exchange reached the 16 GB limit.
• 5.3.2 System not accepting network messages. Look outside Exchange for a connectivity problem.
• 5.3.3 Remote server has insufficient disk space to hold email. Check SMTP log.
• 5.3.4 Message too big. Check limits, System Policy, connector, virtual server.
• 5.3.5 Multiple Virtual Servers are using the same IP address and port. See Microsoft TechNet article: 321721 Sharing SMTP. Email probably looping.
• 5.4.0 DNS Problem. Check the Smart host, or check your DNS. It means that there is no DNS server that can resolve this email address. Could be Virtual Server SMTP address.
• 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 Cannot find the next hop, check the Routing Group Connector. Perhaps you have Exchange servers in different Routing Groups, but no connector.
• 5.4.6 Tricky looping problem, a contact has the same email address as an Active Directory user. One user is probably using an Alternate Recipient with the same email address as a contact.
• 5.4.7 Delivery time-out. Message is taking too long to be delivered.
• 5.4.8 Microsoft advise, check your recipient policy. SMTP address should be cp.com NOT server.cp.com.
• 5.5.0 Requested action not taken: mailbox unavailable. Underlying SMTP 500 error. Our server tried ehlo, the recipient’s server did not understand and returned a 550 or 500 error. Set up SMTP logging.
• 5.5.2 Possibly the disk holding the operating system is full. Or ‘Requested mail action aborted: exceeded storage allocation’ if you are executing an SMTP.
• 5.5.3 More than 5,000 recipients. Check the Global Settings, Message Delivery properties. Or ‘Requested action not taken: mailbox name not allowed’ if you are executing an SMTP.
• 5.5.4 Transaction failed
• 5.5.5 Wrong protocol version
• 5.6.3 More than 250 attachments.
• 5.7.1 Permissions problem. For some reason the sender is not allowed to email this account. Perhaps an anonymous user is trying to send mail to a distribution list. Check SMTP Virtual Server Access Tab. Try checking this box: Allow computers which successfully authenticate to relay. User may have a manually created email address that does not match a System Policy.
• 5.7.2 Distribution list cannot expand and so is unable to deliver its 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 to high.
• 5.7.7 Message integrity problem.