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SMTP, or Simple Mail Transfer Protocol, is a widely used communication protocol that is used to transfer electronic mail messages between servers. The protocol works by defining how servers should communicate with each other to deliver emails from one address to another.

However, as with any protocol, there are bound to be errors and problems that arise during the communication process. To help diagnose and troubleshoot these issues, SMTP defines a set of error codes that are used to communicate the nature of the error to the sender and receiver of the email.

In this blog, we will be discussing SMTP error codes in detail.

SMTP Error Codes

SMTP error codes are three-digit numbers that are used to indicate the status of the email transmission process. They are divided into five categories:

  1. 1xx – These are informational messages that indicate that the server has received the request and is processing it.
  2. 2xx – These are success messages that indicate that the request was successful and that the email has been accepted for delivery.
  3. 3xx – These are temporary errors that indicate that the server is not able to process the request at the moment, but the request can be retried later.
  4. 4xx – These are permanent errors that indicate that the request was not successful and that the email cannot be delivered.
  5. 5xx – These are permanent errors that indicate that the server has encountered a fatal error and cannot process the request.

Let us now take a closer look at each of these categories and the error codes that fall under them.

1xx – Informational Messages

The 1xx category includes messages that are used to indicate that the server has received the request and is processing it. These messages are not considered errors, and the email transmission process will continue.

The following are some of the error codes that fall under this category:

  • 100 – The server has received the request and is continuing to process it.
  • 110 – The server has opened a connection to the client and is waiting for the client to send a command.
  • 120 – The server is ready to transmit data.

2xx – Success Messages

The 2xx category includes messages that indicate that the request was successful and that the email has been accepted for delivery. These messages are not considered errors, and the email transmission process will continue.

The following are some of the error codes that fall under this category:

  • 200 – The server has accepted the request and is ready to process it.
  • 211 – The server is responding to a request for information about its capabilities.
  • 214 – The server is sending information about its capabilities.
  • 220 – The server is ready to accept a connection from the client.
  • 221 – The server is closing the connection.
  • 250 – The server has completed the request and has accepted the email for delivery.

3xx – Temporary Errors

The 3xx category includes messages that indicate that the server is not able to process the request at the moment, but the request can be retried later. These errors are temporary, and the email transmission process will continue after a period of time.

The following are some of the error codes that fall under this category:

  • 300 – The server is ready to accept a message but requires more information from the client before it can proceed.
  • 354 – The server is ready to accept data and is waiting for the client to send it.

4xx – Permanent Errors

The 4xx category includes messages that indicate that the request was not successful and that the email cannot be delivered. These errors are permanent, and the email transmission process will not continue unless the issue is resolved.

The following are some of the error codes that fall under this category:

  • 421 – The server is not available and is shutting down.
  • 450 – The requested action was not completed because the mailbox is unavailable.
  • 451 – The server encountered a temporary error and is unable to process the request at the moment.
  • 452 – The requested action was not completed because the server does not have enough storage space.

5xx – Fatal Errors

The 5xx category includes messages that indicate that the server has encountered a fatal error and cannot process the request. These errors are permanent, and the email transmission process will not continue unless the issue is resolved.

The following are some of the error codes that fall under this category:

  • 500 – The server encountered a syntax error and is unable to process the request.
  • 501 – The requested action is not supported by the server.
  • 502 – The server encountered a command that is not implemented.
  • 503 – The server encountered a command that is out of sequence.
  • 504 – The server encountered a command that requires more information than is currently available.
  • 550 – The requested action was not completed because the mailbox is unavailable or does not exist.
  • 551 – The recipient is not authorized to receive email.
  • 552 – The requested action was not completed because the server does not have enough storage space.

How to Resolve SMTP Errors?

SMTP errors can be frustrating, but they can usually be resolved with some troubleshooting. Here are some tips on how to resolve SMTP errors:

  • Check the email address: Make sure that the email address is correct and that there are no typos or errors in the address.
  • Check the server settings: Make sure that the server settings are correct and that the server is configured to allow email traffic.
  • Check the SMTP port: Make sure that the SMTP port is open and that there are no firewalls or other security measures that are blocking the traffic.
  • Check the message size: If you are receiving a 552 error, check the size of the message and try sending a smaller message.
  • Check the server status: If you are receiving a 421 error, check the status of the server and try again later.

In a Nutshell

Conclusion SMTP error codes are an essential part of the email transmission process. They help diagnose and troubleshoot issues that arise during the communication process. Understanding these error codes can help you identify and resolve issues that may prevent your email from being delivered.

If you are experiencing SMTP errors, don’t panic. Take the time to troubleshoot the issue, and you will likely be able to resolve it with some patience and persistence.

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Eric Lee

A reader, writer, researcher, and explorer!

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