Seen any “Delivery Status Notification” messages lately? With many companies (especially AOL, GMail, and Yahoo) upgrading mail servers or updating their spam filters frequently we’ve seen a big increase in how many outgoing emails are returned to us. Still, it could be your own SMTP, DNS, or anti-spam settings that are causing the problem. To be sure, check the retrun and ESMTP status codes that are in the top few lines of the returned message.

Here’s and example: PERM_FAILURE: SMTP Error (state 8 ): 554 5.1.0 Sender Denied

In this case it’s easy to see that the message was returned because of an SMTP error. The “Sender Denied” comment on the end means it’s probably because the sending mail server was blacklisted by a spam filter. In most cases it’s not that clear and you need to use the status codes to figure out what happened. The status code in this message is the “554 5.1.0″ part. The format is always *** x.x.x, with the x.x.x part being a return code and the *** being an ESMTP status code. A chart of their meanings is below:

x.x.x, Server Return Codes:
The first value indicates the status of the communication between the sender and recipient server. Possible values are:

• 1 - Server accepted the command, but no action taken. Confirmation message is required.
• 2 - Server successfully completed the task.
• 3 - Server understood the request. More information is required to complete.
• 4 - Server encountered a temporary failure on that request. The command may complete if it’s repeated.
• 5 - Server encountered an error.

The second value is an extra level of detail, which indicates the type of error or communication:

• 0 - Syntax error has occured.
• 1 - Message is an informational reply.
• 2 - Message refers to the connection status.
• 3 - Unspecified, may be used for custom messages.
• 4 - Unspecified, may be used for custom messages.
• 5 - Status message refers to the entire mail system or server.

***, ESMTP Status Codes:
In the event of a failure ESMTP codes can indicate even more detail than server return codes. They can indicate status of either the recipient mail server or your local mail server in response to a problem with your mail client. Possible values are:

• 211 - System status message.
• 214 - Help message formatted for human reader follows.
• 220 - SMTP service ready.
• 221 - Service/connection closing.
• 250 - Successful request. Action completed.
• 251 - Recipient is not local to the server, but the server will accept and forward the message.
• 252 - Recipient can’t be verified, but the server will accept the message and attempt delivery.
• 354 - Start message input now, end with <crlf>.</crlf>. Indicates the server is ready to accept a message once you’ve given it From: and To: information
• 421 - Service is not available and connection will be closed.
• 450 - Requested command failed because the recipient’s mailbox is unavailable.
• 451 - Command has been aborted due to a server error. Possibly notify your SysAdmin.
• 452 - Command has been aborted because the server has insufficient system storage.
• 500 - Server could not recognize the command was due to a syntax error. (usually due to mail client error)
• 501 - Syntax error was found in command arguments. (usually due to mail client error)
• 502 - Command was not implemented. (usually due to mail client error)
• 503 - Server has encounterd a bad command or sequence of commands. (usually due to mail client error)
• 504 - Command parameter is not implemented. (usually due to mail client error)
• 550 - Command failed because the user’s mailbox was unavailable (or you did not have permissions to send to this mailbox)
• 551 - Recipient is not local to the server. Server responds with a fowarding address that should be tried.
• 552 - Action was aborted because storage allocation was exceeded.
• 553 - Action was aborted because the mailbox name was invalid.
• 554 - Transaction failed, without a clear reason.

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