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SMTP: Analysis of common errors, error codes and meanings

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deliverability

SMTP introduction

When you send an e-mail, you may encounter errors that prevent the message from being delivered. These errors are often indicated by error codes SMTP that provide information about the nature of the problem. Understanding the operation of these error codes is essential for diagnosing and resolving messaging problems and deliverability. This article reviews the most common SMTP errors, their meanings and possible solutions.

1 - Introduction to SMTP Error Codes

SMTP error codes are responses generated by the SMTP servers to indicate the status of a message. They appear on Snapshoot in the form of three-digit numbers, where each digit has a specific meaning:

  • The first number indicates the class of the response:

    • 2xx : Successful (order accepted)
    • 4xx : Temporary error (try again later)
    • 5xx Permanent error (command failed)
 

  • The second number indicates the error category:

    • x0x : Syntax error
    • x1x Status information
    • x2x : Connections
    • x3x E-mail address
    • x4x : Message service
    • x5x Mailbox status
 

  • The third number further specifies the SMTP error.

2 - Common SMTP errors and what they mean

4xx errors: temporary errors, soft bounce

These errors signal a temporary problem and indicate that the sender should try again later. The SMTP protocol is very precise and presents a status for each email sent. 

  • 421 Service Not Available

    • Meaning: Service is unavailable, often due to overload or maintenance.
    • Solution: Wait a moment and try again. If the problem persists, contact the SMTP server administrator.

  • 450 Requested Mail Action Not Taken: Mailbox Unavailable

    • Meaning: Inbox temporarily unavailable, probably full.
    • Solution: Try again after a while. If the problem persists, ask the recipient to check their inbox space.
 

  • 451 Requested Action Aborted: Local Error in Processing

    • Meaning: A local error prevented the message from being processed.
    • Solution: Wait a moment and try again. Also check your server for technical problems.
 

  • 452 Requested Action Not Taken: Insufficient System Storage

    • Meaning: The server has storage problems, often due to a full inbox.
    • Solution: Try again later. If the problem persists, contact the server administrator.
 

5xx errors: permanent errors, hard bounce

These errors indicate a definitive failure and usually require intervention to correct the problem.

  • 500 Syntax Error, Command Unrecognised

    • Meaning: The command sent to the SMTP server contains a syntax error.
    • Solution: Check the command sent. There could be a typo or incorrect configuration.
 

  • 501 Syntax Error in Parameters or Arguments

    • Meaning: The command parameters or arguments are incorrect.
    • Solution: Check the arguments given in the SMTP command.
 

  • 502 Command Not Implemented

    • Meaning: The command is not supported by the SMTP server.
    • Solution: Make sure you're using server-compatible commands.
 

  • 503 Bad Sequence of Commands

    • Meaning: The orders were sent in the wrong order.
    • Solution: Check and correct the order of SMTP port commands.
 

  • 504 Command Parameter Not Implemented

    • Meaning: The command parameter is not supported by the server.
    • Solution: Recheck port control settings.
 

  • 550 Requested Action Not Taken: Mailbox Unavailable

    • Meaning: The recipient's inbox is unavailable, often because the email address is incorrect.
    • Solution: Check the recipient's email address and try again.
 

  • 551 User Not Local; Please Try Forwarding

    • Meaning: The recipient is unknown on this server, but could be found elsewhere.
    • Solution: Check the email address and, if necessary, contact the recipient for a valid address.
 

  • 552 Requested Mail Action Aborted: Exceeded Storage Allocation

    • Meaning: The recipient's inbox has exceeded its storage capacity.
    • Solution: Ask the recipient to free up space or use another address.
 

  • 553 Requested Action Not Taken: Mailbox Name Not Allowed

    • Meaning: The recipient's email address is syntactically incorrect.
    • Solution: Correct the email address and try again.
 

  • 554 Transaction Failed 

    • Meaning: A general error caused the transaction to fail.
    • Solution: Recheck all email parameters and try again. If the problem persists, contact the server administrator.

Find out more

You can test the SMTP service by opening a free test account with 2000 free e-mails per month at Snapshoot here.

Or take a look at the documentation techniques of our company and find out more about blog.

Thanks for reading!

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