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What are SMTP error codes and which are the most common?

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1 year ago

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The Simple Mail Transfer Protocol (SMTP) is the standard protocol for transferring emails between computers. When you type an email and hit send, you’re instructing your mail server to transfer the message contents to the recipient’s server, and the SMTP protocol guides this data transfer.

However, things do not always go without a hitch, and the transfer may fail due to an error. This article will tell you what you need to know about SMTP error codes.

What Are SMTP Error Codes?

The SMTP protocol uses reply codes to share server status updates. When you issue a command to an SMTP server, you’ll receive a three-digit code in response. The code tells you if the server understood and executed your request. If the server fails to execute your request, you’ll receive an error message with a three-digit code indicating the specific error.

Response codes beginning with a 2 or 3 indicate a positive response. Anything beginning with a 4 or 5 indicates an error.

  • 2xx response code means the command was executed successfully.
  • 3xx indicates the server has accepted the request but put the operation on hold, awaiting further information.
  • 4xx means the request couldn’t be completed because of a transient error, and may be successful on retry.
  • 5xx means the request couldn’t be executed because of a permanent error needing further attention.

The second digit of an error code gives a hint of what it’s about.

  • x0x indicates a syntax error wherein the server received a non-existent command.
  • x1x indicates an information reply, e.g., a user requests help, and the server responds with the link to the help center.
  • x2x implies a connection issue because of hardware or software faults. The issue is usually temporary and auto-resolves if you resend the command later.
  • x3x and x4x are unspecified errors, which can understandably be frustrating.
  • x5x means the mail system is responsible for the issue, e.g., the recipient’s inbox doesn’t exist.

The third digit is used to distinguish between different errors of the same type.

One important thing about SMTP error codes is that the implementation is not always consistent across different SMTP applications with regard to the second and third digits.

Error Code Examples 

Let’s look at some common SMTP errors you may encounter:

Code Meaning Possible fixes
421 Service not available. It’s usually a temporary error that occurs when the recipient’s server gets shut down. Try using another SMTP port.

Ask the recipient to whitelist your email server.
442 The connection was terminated in transmission. It usually occurs because of Internet connection issues. Try resending the message.

If the problem persists, contact your internet service provider (ISP) for help.
550 Inbox unavailable. The recipient’s email address does not exist. Check for possible typos in the receiving address.
554 Transaction failed. This usually occurs if the receiving server has blocked your server’s IP address. Check if your IP address is blacklisted. If so, contact the blacklist owner for delisting.
500 Syntax error. The command does not exist. Restart the SMTP service.

Temporarily disable firewall or antivirus tools because they might be interfering with the connection.
523 Mailing list size exceeded: too many recipient addresses. Break the mailing list into smaller batches and send the emails at intervals.

How To Avoid SMTP Errors

Here are useful tips to help you avoid errors:

  1. Double-check the receiving address for typos before hitting the send button. Also, ensure any attachment falls within your SMTP provider’s size limit.
  2. Limit the sending volumes to what your email service provider (ESP) allows. Sending past your allocated limit will definitely cause errors.
  3. Choose a reputable email service provider with secure and reliable servers, e.g., UniOne.
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