Understanding SMTP Error Messages Print

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Way too often, error messages are but mystic numbers forming some kind of incomprehensible
code. This page wants to be your guide to the code mail servers produce when you
try to send mail, but it doesn't work.

"Could not send your message. Error 421."

This was all the email client showed her. This, and a mere button waiting for
the mouseclick 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 occured, 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. This is hardly ever used by mail

5: The server has encountered an error.


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


0: A syntax error has occured.

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

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 layed down in RFC 821 and later extensions.


List of SMTP Server Response Codes

211 - A system status message.

214 - A help message for a human reader follows.

220 - SMTP Service ready.

221 - Service closing.

250 - Requested action taken and completed. The best message of them all.

251 - The recipient is not local to the server, but the server will accept and
forward the message.

252 - The recipient cannot be VRFYed, but the server accepts the message and
attempts delivery.

354 - Start message input and end with <CRLF>.<CRLF>. This indicates
that the server is ready to accept the message itself (after you have told it
who it is from and where you want to to go).

421 - The service is not available and the connection will be closed.

450 - The requested command failed because the user's mailbox was unavailable
(for example because it was locked). Try again later.

451 - The command has been aborted due to a server error. Not your fault. Maybe
let the admin know.

452 - The command has been aborted because the server has insufficient system


The following error messages (500-504) usually tell you that your email
client is broken. It's probably best to let the program's author know.


500 - The server could not recognize the command due to a syntax error.

501 - A syntax error was encountered in command arguments.

502 - This command is not implemented.

503 - The server has encountered a bad sequence of commands.

504 - A command parameter is not implemented.

550 - The requested command failed because the user's mailbox was unavailable
(for example because it was not found, or because the command was rejected for
policy reasons).

551 - The recipient is not local to the server. The server then gives a forward
address to try.

552 - The action was aborted due to exceeded storage allocation.

553 - The command was aborted because the mailbox name is invalid.

554 - The transaction failed. Blame it on the weather.


Reprinted from - http://email.about.com/cs/standards/a/smtp_error_code_p.htm



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