Understanding SMTP Error Codes

Way too often, error messages are incomprehensible. This page will be your guide to the code mail servers produce when your email fails to send. If you receive an error message like, "Could not send your message. Error 421," what's your next step? Let this page be your guide to what to do next.

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SMTP Error Codes: the Meaning Behind 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 indicates 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. It's six possible values are:

  • 0: A syntax error has occurred.
  • 1: Indicates an 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.

The most common SMTP error code when sending email is 550.

SMTP error 550 is a generic error message. It means the email could not be delivered.

An SMTP error 550 delivery failure happen for a variety of reasons; while the error code 550 itself tells you nothing about the cause of failure, many an SMTP server includes an explanatory message with the error code.

Variations of SMTP 550

Often, an email could not be delivered because it has been blocked as spam, either through an analysis of its contents or because the sender—or sender's network—is listed as a probable source of spam in a DNS blocklist. Some mail servers check for links to malware as well and return an error 550. SMTP error 550 codes for these cases include:

  • 550 5.7.1: Service unavailable: client [###] blocked using ### (Exchange Server)
  • 550 5.7.1: Message rejected as spam by Content Filtering (Exchange Server)
  • 550 This message was classified as SPAM and may not be delivered
  • 550 High probability of spam (Gmail)
  • 550 5.2.1 mail from refused spam site
  • 550 Your message has been rejected because you have been detected sending spam (Sending from Rackspace)
  • 550 Message contained unsafe content

What can you do? If possible, try to contact the recipient by other means. If the error message points to a specific blocklist or spam filter, do try to contact the list or filter administrator. Failing all this, you can always explain the unfortunate situation to your email provider. They may be able to contact their colleague at the receiving end and get the situation sorted.

List of SMTP Error Codes (With Explanations)

An SMTP error's three numbers get us a detailed list of ESMTP/SMTP server response codes, as laid down in RFC 821 and later extensions:

  • 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 .. 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.
  • 455 - The server cannot deal with the command at this time.

Got SMTP 550: Permanent Failure for One or More Recipients?

The following error messages (500-504) usually tell you that your email client is broken or, most commonly, that your email could not be delivered for one reason or another.

  • 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.
  • 521 - This host never accepts mail; a response by a dummy server.
  • 541 - The message could not be delivered for policy reasons—typically a spam filter. (Only some SMTP servers return this error code.)
  • 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.
  • 555 - The server does not recognize the email address format, and delivery is not possible.
  • 556 - The message would have to be forwarded, but the receiving server will reject it.
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