21 SMTP Response Codes

The SMTP response code list can be used to help quickly determine the reason for email bounces or why you received an SMTP error when sending an email.

Kuldeep Chhipa
Kuldeep Chhipa
August 09, 2021
5 min read

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The SMTP response code list can be used to help quickly determine the reason for email bounces or why you received an SMTP error when sending an email.

Therefore, in this blog post, we will provide you with the 21 most common SMTP response codes.

What is SMTP Response Code

The recipient’s mail server returns a code indicating the status of the mail. This code is called the SMTP Response Code.

SMTP Response Code consists of three digits:

  1. The error code class: The first number indicates whether the server accepts the command. There are five possible values, ranging from 1 to 5. Each value determines whether the server accepted the command, performed an action, had a temporary problem, or had an error.
  2. Subject: The second digit of the SMTP response code provides additional information and indicates whether there is a syntax, communication, or other problem.
  3. Detail: The last digit provides more information about the mail transfer status.

Email Monitoring Response codes

  1. 220 — SMTP Service ready. This is a general response, indicating that the SMTP server is ready to proceed to the next command.
  2. 221 — Service closing. The session or connection of the mail server is ending and all processes are completed. The result may be a Pass or Fail.
  3. 250 — Requested action taken and completed. This is the best message the sender can receive because it indicates that the SMTP communication was successful. The SMTP 250 response code is also the most common in SMTP because it is issued in response to each received command (each message maybe 4-6 times).
  4. 354 — Start message input and end with … Indicates that the server is ready to accept the message itself.
  5. 421 — The service is not available, the connection will be closed. If you receive an “SMTP error (421) Failed to connect to the server”, this is considered an open error, which is usually related to the “unreachable” target server. However, this may also mean that your outgoing mail server is “unreachable.” If you use a remote mail server to send mail.
  6. 450 — The requested command failed because the user’s mailbox was not available There also can be some additional meanings to the present response code such as:

    1. the e-mail account does not exist on the recipient mail server
    2. the e-mail account doesn’t have permission to receive the email
    3. The recipient mail server rejected the email thanks to blacklisting or filtering

    How to Fix SMTP Error 450:

    1. Verify that the email account you are trying to access exists Check the IP address of your mail server to see if it is blacklisted.
    2. We strongly recommend MXToolbox run a blacklist check.
    3. If your email appears to be spam, please delete all unwanted content from the message.
    4. If you are using attachments, try sending an email without attachments.
  7. 451 — Aborted due to a server error. It is generally not your responsibility as the recipient's mail server rules may interfere with the processing of your email.
  8. 452 — The server has insufficient system storage. This usually happens because the mail server is overloaded when sending multiple emails at the same time. To resolve this issue, check your mail queue and use the correct delivery rate.
  9. 455 — The server cannot interact with the command at this time. If you receive this message, please allow time to try more. If unsuccessful, please contact the administrator of the recipient’s mail server.
  10. 500 — The server could not recognize the command because of a syntax error. Your answer may be due to antivirus software or firewall. To confirm this is the case, try disabling each function to test the connection.
  11. 501 — A syntax error occurred in command arguments. This is similar to the SMTP 500 response code. However, 501 responses are usually caused by incorrect or invalid email addresses.
  12. 502 — This command is not implemented. If you get this answer, there may be a problem with your original MTA configuration.. Check your MTA settings and try to connect again.
  13. 503 — Bad sequence of commands at server side. This response indicates that the parameters used do not match the order expected by the mail server, which usually occurs when the email account is not authenticated. To resolve this error, please verify that your email account is properly authenticated and make sure that all MTA settings are correct.
  14. 504 — A command parameter is not implemented. Same like response code 502.
  15. 521 — This host never accepts mail, a response by a dummy server. This response simply means that the recipient’s mail server did not accept or deliver the email. To correct SMTP error 521, verify that the message failed and was not retransmitted.
  16. 541 — Message not delivered because of policy reasons — If you receive this SMTP response, it means that your email may have been marked as spam. Confirm that your server is not sending spam and is not blacklisted.

SMTP Email Status Code 500

  1. 550 — Request failed because Mailbox did not activate For example, In addition, the SMTP 550 response code is often used to indicate other instances of permanent failure. For example, “The 550 mail server has detected that your mail is spam and has blocked it from being sent.”
  2. 551 — The receiving server is not local to the server. Then the server gives a forwarding address for testing. This is often used as a spam prevention strategy.
  3. 552 — Exceeded storage allocation. This is usually because the recipient’s mail server is too full. This may be because the recipient did not check their email, or in some more extreme cases, the recipient was a victim of mail bombing.
  4. 553 — The mailbox name is invalid. In this case, the mailbox cannot verify the email address. Please check to make sure all email addresses you send to are correct.
  5. 554 — delivery error: Sorry, your message cannot be delivered. This mailbox is disabled. If you receive the SMTP code 554, this is just a normal invalid address response.

What to do when response code occurs

First of all, don’t panic!

SMTP response codes are just a simple way for servers to communicate with each other.

If you receive an SMTP response, the first step is to ask yourself why you received the response. This usually starts with simple research. For research purposes, we strongly recommend that you bookmark this page for quick and easy reference when encountering SMTP response codes in the future.

In most cases, the email response code is not required to take any action; however, if an SMTP error occurs due to an invalid or permanent failure of the address, it is best to remove the failed address from the email list, So you don’t do this. Repeatedly sent to the same wrong email address in the future. This is because sending to the wrong email address can damage the sender’s reputation.



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Kuldeep Chhipa

Kuldeep Chhipa

Kuldeep is a Senior Quality Assurance Engineer and loves Automation. He helps business owners to grow their businesses online.

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