What is SMTP reply?

SMTP, or Simple Mail Transfer Protocol, is the standard protocol for sending emails across the internet. When an email client or server sends a request to another SMTP server, that server sends back a reply code indicating whether the request was successfully received and processed. Understanding SMTP reply codes is important for efficiently troubleshooting email delivery issues.

What is SMTP reply?

How SMTP Works

Here is a quick overview of how SMTP works:

  • A user composes an email in their email client (Outlook, Gmail, etc.).
  • The email client connects to the sender’s SMTP email server.
  • The sending SMTP server opens a connection with the recipient SMTP server.
  • The sending SMTP transfers the email content.
  • The receiving SMTP server issues SMTP reply codes indicating status.
  • If successful, the email gets stored in the recipient’s mailbox.

So SMTP reply codes allow the sending and receiving servers to communicate status for optimization.

Categories of SMTP Reply Codes

SMTP response codes are three-digit numbers that start with a digit indicating the broader meaning:

  • 1xx – Positive preliminary reply
  • 2xx – Positive completion reply
  • 3xx – Positive intermediate reply
  • 4xx – Temporary negative reply
  • 5xx – Permanent negative reply

The first digit dictates the general meaning of the response. The later two digits provide more nuanced meanings. By learning the various codes, you can better optimize communications.

Common SMTP Reply Codes

Here are some of the most common SMTP response codes:

Code Meaning
200 Nonstandard success response
211 System status, or system help reply
214 Help message response
220 Service ready
221 Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward
252 Cannot verify user, but will try to deliver
354 Start mail input; end with <CRLF>.<CRLF>
421 Service not available, closing transmission channel (This may be a reply to any command if the service knows it must shut down)
450 Requested mail action not taken: mailbox unavailable
451 Requested action aborted: error in processing
452 Requested action not taken: insufficient system storage
500 Syntax error, command unrecognized (This may include errors such as command line too long)
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
521 Domain does not accept mail (See RFC 1846)
530 Access denied
550 Requested action not taken: mailbox unavailable (For example, mailbox not found, no access, or command rejected for policy reasons)
551 User not local; please try <forward-path>
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed (For example, mailbox syntax incorrect)
554 Transaction failed

This covers the most common codes, but there are many more specific ones as well.

Optimizing Based on SMTP Reply Codes

If you get familiar with these codes, you can often resolve email delivery issues right away. Some key things to check based on certain codes:

  • 400 range – Temporarily unavailable. Check connectivity and try again later.
  • 500 range – Permanent error for sender. Check for syntax errors, unregistered users, etc.
  • 200, 250 – Successful email accepted from sender. No action needed.
  • 450, 550 – Permanent error for recipient. Check if mailbox disabled, not found, etc.
  • 252, 451 – Grey area errors. May be accepted but issues with recipient mailbox. Review closely.

Adjusting variables like server connections, authentication settings, email content formatting, etc. based on the specific SMTP response can get your emails reliably delivered.

SMTP Reply Code Lookup Tools

Manually checking all codes is tedious. Thankfully, there are some great reference tools:

These make it easy to instantly check the meaning of any 3-digit SMTP status response.

Key Takeaways on SMTP Reply Codes

  • SMTP response codes indicate status when email servers communicate.
  • The first code digit indicates a broader meaning – positive, temporary error, permanent error, etc.
  • Specific codes have defined meanings to optimize troubleshooting.
  • Tools like lookup tables help easily reference any confusing codes.
  • Learning common codes saves time resolving email delivery problems.


In summary, SMTP reply codes allow sending and receiving email servers to communicate about the status of message transmission. While the many 3-digit codes may seem confusing at first, learning the common responses and meanings enables quick troubleshooting. If issues come up, consult an SMTP response code reference to optimize delivery based on the specific situation. With practice, you will know how to tweak settings to reliably send emails based on the reply message.

Frequently Asked Questions about SMTP Reply Codes

Here are answers to 20 common questions about SMTP response codes:

  1. What is an SMTP reply code?
    An SMTP reply code is a 3-digit status code that SMTP email servers send in response to requests. These codes indicate if the request was received, if there was an error, or other statuses.
  2. What do the digits in an SMTP code mean?
    The first SMTP code digit gives the general status:

    • 1xx = Positive response
    • 2xx = Successful response
    • 3xx = Positive intermediate response
    • 4xx = Temporary error
    • 5xx = Permanent error
  3. The second two digits provide more nuanced specific statuses.
  4. What does SMTP code 250 mean?
    Code 250 means the requested action was completed successfully. It indicates an email was accepted for delivery.
  5. What does SMTP code 553 mean?
    Code 553 indicates the action was rejected due to an invalid mailbox name. Common causes are mailbox syntax errors or trying to send to a nonexistent account.
  6. What does SMTP code 451 mean?
    Code 451 means the action was aborted due to an error processing the request on the server side. There may be a configuration issue causing problems.
  7. What does SMTP code 221 mean?
    Code 221 means the service is closing the connection channel, indicating the transaction completed.
  8. What does SMTP code 421 mean?
    Code 421 signals that the service is unavailable and closing the connection. This could happen if an SMTP server is taken offline.
  9. What does SMTP code 451 mean?
    Code 451 means the action was aborted due to a server side processing error. There may be a misconfiguration causing problems.
  10. What does SMTP code 550 mean?
    550 signifies a permanent error that the requested mailbox is unavailable. Common causes include disabled accounts or entering an invalid mailbox.
  11. What does SMTP code 554 mean?
    Code 554 means the mail transaction failed for some reason. Troubleshooting the specifics around why it failed is recommended.
  12. What does SMTP code 503 mean?
    A 503 status code signifies an invalid sequence of client commands sent to the server. Resending with valid syntax should resolve it.
  13. What is the difference between SMTP code 500 vs 501?
    • 500 means invalid syntax, like an unrecognized command
    • 501 specifically refers to syntax errors in the parameters and arguments
  14. What is SMTP code 502 and 504?
    • 502 means the client sent a command that the server does not implement
    • 504 similarly indicates the command parameter is not implemented/supported.
  15. Can an SMTP server connection start with errors?
    Yes, it is possible to receive 4xx and 5xx errors at initial connection if the service is down, authentication fails, etc., before sending any actual email.
  16. Where can I lookup confusing SMTP reply codes?
    Sites like Toolbox have helpful SMTP code lookup tables where you can search 3-digit codes to understand their meaning. There are also SMTP test tools to validate server configurations.
  17. How long has SMTP used 3-digit reply codes?
    SMTP reply codes have been part of the core SMTP standard RFC 821 since 1982 when SMTP was originally published. The convention helps standardized troubleshooting.
  18. Do all SMTP servers use the same reply codes?
    While the IETF dictates standard codes in the SMTP RFCs, some mail systems do use proprietary codes. But most will follow the standard for compatibility.
  19. Why does SMTP use numeric codes vs text statuses?
    SMTP was created in 1982 before ubiquitous broadband. Minimizing data transfer with lightweight numeric codes helped efficiency over slow older networks.
  20. Can I tell if an email was delivered based on SMTP codes?
    You can narrow chances through inferences, but cannot guarantee delivery. For example, 250 means accepted by next server, but factors like full mailbox quota can still later bounce it.
  21. Will understanding SMTP codes help me resolve email issues?
    Absolutely. When troubleshooting delivery issues, recognizing the meaning of different SMTP reply codes lets you pinpoint configuration changes needed to optimize successful send outs.

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