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“Could not send your message. Error 421.” – Meaning of the Numbers

Have you ever tried wondering what the codes actually mean? Or have you thought, if these numbers do make some sense? Or if they might mean something which could help you identify with the issue. Very often you encounter this in your mailbox. I intend just giving a small heads-up in understanding the meaning or interpreting the error codes which might help in making sense as to where the trouble could be.

Primarily you will get the error code with three numbers as I showed above Error 421.

First Number: This lets you know whether the server has accepted the command and was capable to handle it or not. Let me address you the possible meanings of the first number in the error now.

1: The server has taken the command, but it’s not executing it. Here you need a confirmation message.

2: The server was able to complete the task.

3: The server was able to understand the request but feels it needs more information to complete it.

4: The server faces a temporary failure here. If you try executing the command again, it may complete. At times, mail servers use such temporary failures to keep security higher.

5: The server actually has the error and you should attempt it.

Second Number: This gives a little more information. Let me address you the possible meanings of the second number in the error now:

0: This indicates that a syntax error has taken place.

1: This indicates an informational reply, for example to a HELP request.

2: This indicates your connection status.

3 and 4 are unspecified.

5: This indicates your status of the mail system as a whole and the mail server in particular.

Third Number: The last number is the key. It pertains more to mail transfer status. This actually relates to the detailed list of ESMTP server response codes, as laid down in RFC 821 and later extensions.

211 – This indicates a system status message.

214 – This indicates that a help message for a human reader follows.

220 – This indicates that SMTP Service ready.

221 – This indicates that service is closing.

250 – This indicates that requested action taken and completed.

251 – This indicates that the server will accept and forward the message though the recipient is not local to the server.

252 – This indicates the server accepts the message, tries delivering it inspite of the recipient not being VRFYed.

354 – Start message input and end with <CRLF>.<CRLF>. This indicates that the server is ready to accept the message.

421 – This indicates that the service is not available and the connection will be closed.

450 – This indicates to try again later as the requested command failed due to unavailability of user’s mailbox.

451 – This indicates server error.

452 – This indicates insufficient system storage.

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 – This indicates syntax error.

501 – A syntax error was encountered in command arguments.

502 – This command is not implemented.

503 – This indicates that the server has encountered a bad sequence of commands.

504 – This indicates that command parameter is not implemented.

550 – This indicates that 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 – This indicates that storage allocation was exceeded.

553 – This indicates invalidity of the mailbox name.

554 – The transaction failed.


Ratish Nair
MVP Exchange
Team @MSExchangeGuru

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