Understanding SMTP Error Messages

"Could not send your message. Error 421."

This was all the email client showed her. This, and a mere button waiting for the mouse click of relief that would make even the sparse message go away: OK.

No, it is not OK. It is not okay to leave the user with a mystic code that tells nothing about whose fault it was that the message could not be sent, which error exactly occurred, and, in particular, what can be done to resolve the problem.

It is not okay, but only too often it happens. What if all a physician said was: "we have to do a hypopharyngoscopy"?

What's your next step? You consult somebody who you think knows, a clinical dictionary or the web. This page wants to be your dictionary for (E)SMTP (the protocol used to send emails) server response codes.

The Meaning of the Numbers

A mail server will reply to every request a client (such as your email program) makes with a return code. This code consists of three numbers.

The first generally tells whether the server accepted the command and if it could handle it. The five possible values are:

• 1: The server has accepted the command, but does not yet take action. A confirmation message is required. Currently, this is not used.
• 2: The server has completed the task successfully.
• 3: The server has understood the request, but requires further information to complete it.
• 4: The server has encountered a temporary failure. If the command is repeated without any change, it might be completed. Mail servers can use such temporary failures to keep untrusted senders at bay.
• 5: The server has encountered an error.

The second number gives more information. Its six possible values are:

• 0: A syntax error has occurred.
• 1: Indicates a informational reply, for example to a HELP request.
• 2: Refers to the connection status.
• 3 and 4 are unspecified.
• 5: Refers to the status of the mail system as a whole and the mail server in particular.

The last number is even more specific and shows more graduations of the mail transfer status. This leads us to the detailed list of ESMTP server response codes, as laid down in RFC 821 and later extensions.