Common SMTP Error Codes

No idea what “Could not send your message. Error 421.” means? You’re not alone! How can you determine if an email is returned due to an invalid email address on the recipient’s part, SPAM filtering or a problem with your email server? Good question!

SMTP (Simple Mail Transfer Protocol), is the protocol used to send email. What happens when you “Send/Receive”? Let’s take a quick look at the process:

1. You compose a new email, add a recipient, add content, preferably proofread your email and then click the magic “Send/Receive” button

2. The email leaves your Outlook Outbox and goes through your email server for directions to the recipient’s email server. It is then forwarded to the email server for the receiver’s domain.

3. Once the recipient’s email server receives your painstakingly constructed and visually appealing email, the email server reads the address of the end-user’s mailbox.

4. The email is then delivered to the recipient’s inbox for review.

What happens when something goes wrong in the process?

You get a return email usually addressed from “System Administrator” that looks something like this:

From: System Administrator
Sent: Tuesday, November 09, 2010 2:59 PM
Subject: Undeliverable:

Your message did not reach some or all of the intended recipients.


Sent:     11/9/2010 2:59 PM

The following recipient(s) could not be reached:

[Recipient’s Name] on 11/9/2010 2:59 PM

There was a SMTP communication problem with the recipient’s email server.  Please contact your system administrator.

< #5.5.0 smtp;550 Requested action not taken: mailbox unavailable>

What on earth failed?

That’s where SMTP error codes come in…

An SMTP error code consists of 3 numbers. The first number displays a status acknowledging whether the server accepted the command and if it had problems handling it. There are five possibilities for the first  of the three numbers:

  • 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 third number denotes a specific error.

The following is a list of common SMTP error 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 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 – 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. Probably a parity error.

We sincerely hope this guide helps to understand the wild world of SMTP.

Happy Sending (and Receiving)!

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