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What SMTP mail error messages mean.
Mailservers error messages, what they mean and what you can do to resolve them.

Sometimes when you try to send an email, the server faces a problem and  can not be delivered the message to the recipient. In this case, you receive a response message stating why your email was not delivered. This message usually includes an error code in order to tell you what happened.

Let see an example bellow:

550 5.7.1 No such user!

The example above can be divided into 3 levels of information:

  • 550 – Basic status code
  • 5.7.1 – Enhanced status code
  • No such user – Reply text

email error explanation

Basic status code

The Basic status code describes a general reason why the error occurred.

In our example, the number 550 matches the base code and shows that the user's mailbox was not found.

Below are the most common basic status codes:

  • 421 - Service unavailable, terminating the connection.
  • 450 - The command was not executed: the mailbox is not available.
  • 451 - Command canceled: error occurred during the execution.
  • 452 - The command was not executed: not available system space.
  • 500 - Syntax error, command not recognized.
  • 502 - The command is not available.
  • 503 - The server encountered a bad set of commands.
  • 504 - A command parameter is not available.
  • 521 - This domain does not accept mail messages.
  • 550 - The user's mailbox is not available (not found).
  • 551 - The user's mailing address is not on this server.
  • 552 - Error because the recipient's mailbox reached it's size limit.
  • 553 - The command was rejected because the user's mailbox name is wrong.
  • 554 - Sending failed permanently, Usually this happens as your message has been flagged as spam or your IP is in the server's black list.

Enhanced status code

In order error messages get more information, the enhanced status codes have been created, consisting of three layers separated by dots.

In our example the number 5.7.1 corresponds to the enhanced code and shows that a critical error occurred (5.x.x) and it's delivery is not allowed (because the user was not found) therefore the message was rejected (x.7.1).

  • The first number explains a general situation:

    • 2.x.x - The server executed the command successfully. For example the email delivered. This is a success code.
    • 4.x.x - The server encountered a temporary error and will try to send the email again until the allowed repeat limit is reached.
    • 5.x.x - The server encountered a critical error. For example the email address was not found, DNS problem or the email stopped by the recipient's server.
  • The second number of the enhanced code gives the subject to which the error refers and the third number explains in detail the error that happened (x corresponds to the number 4 for a temporary error or the number 5 for a critical error):

    • Enhanced codes with number 1 in the second place are relevant to the mailing address status:

      • x.1.0 - Other undefined error related to mailing address status. See reply text if available.
      • x.1.1 - Incorrect mailbox destination address.
      • x.1.2 - Wrong system destination address.
      • x.1.3 - Incorrect syntax in mailbox destination address.
      • x.1.4 - Unclear mailbox destination address.
      • x.1.5 - Valid mailing address.
      • x.1.6 - The mailbox has been moved.
      • x.1.7 - Bad sender's mailbox syntax.
      • x.1.8 - Bad sender's address.
    • Enhanced codes with number 2 in the second place are relevant to the mailbox status:

      • x.2.0 - Other undefined error related to mailbox status. See reply text if available.
      • x.2.1 - The mailbox has been deactivated, it does not accept other messages.
      • x.2.2 - The mailbox is full.
      • x.2.3 - The size of the message exceeds the administrator's limit.
      • x.2.4 - Problem while expanding the mailing list.
    • Enhanced codes with number 3 in the second place are relevant to the status of the mail system:

      • x.3.0 - Other unspecified error related to the status of the mail system. See reply text if available.
      • x.3.1 - The mail system is full.
      • x.3.2 - The system does not accept network messages.
      • x.3.3 - The system does not have the selected feature.
      • x.3.4 - Very big message for the system.
      • x.3.5 - Incorrect system setup.
    • Enhanced codes with number 4 in the second place are relevant to the network and routing status :

      • x.4.0 - Other undefined error related to network or routing status. See reply text if available.
      • x.4.1 - There was no response from the hosting system.
      • x.4.2 - Bad connection.
      • x.4.3 - Error with the routing server.
      • x.4.4 - Routing is not possible.
      • x.4.5 - Network congestion.
      • x.4.6 - Routing loop was detected.
      • x.4.7 - Delivery time is over.
    • Enhanced codes with number 5 in the second place are relevant to the status of a mail delivery protocol:

      • x.5.0 - Other undefined error related to the status of a protocol. See reply text if available.
      • x.5.1 - Invalid command.
      • x.5.2 - Error in command syntax.
      • x.5.3 - Too many recipients.
      • x.5.4 - Invalid command arguments.
      • x.5.5 - Wrong protocol version.
    • Enhanced codes with number 6 in the second place are relevant to the content of the message or the multimedia/files it may contain:

      • x.6.0 - Other unspecified error related to multimedia/files. See reply text if available.
      • x.6.1 - The file is not supported.
      • x.6.2 - File conversion is necessary, but is forbidden.
      • x.6.3 - File conversion is necessary, but is not supported.
      • x.6.4 - File converted with quality loss.
      • x.6.5 - File conversion failed.
    • Enhanced codes with number 7 in the second place are relevant to the server security policy:

      • x.7.0 - Other unspecified error related to server security. See reply text if available.
      • x.7.1 - Delivery not allowed, message was rejected.
      • x.7.2 - The expansion of the mailing list is forbidden.
      • x.7.3 - Security conversion required, but is not allowed.
      • x.7.4 - Security features are not supported.
      • x.7.5 - Encryption error.
      • x.7.6 - The encryption algorithm is not supported.
      • x.7.7 - Message integrity failure.

Reply text

The reply text helps the user to better understand the error.

In our example the text “No such user” corresponds to the reply text and simply states that the user was not found on the server.

Message examples

Below are some message examples that include just the reply text without an error code and some examples from Gmail's servers.

General examples

MessageExplanation
Domain example.gr has exceeded the max defers and failures per hour (x/x (xx%)) allowed. Message discarded Outgoing mail from your domain has exceeded the limit of failed deliveries per hour that are set by the administrator, the mail server refuses to serve you. Try sending your message again after 1 hour.
The mail server could not deliver mail to example@gmail.gr. The account or domain may not exist, they may be blacklisted, or missing the proper dns entries The mail delivery domain does not exist or has not an MX record.
Sender verify failed The sender's account does not exist or the MX records are not set up correctly in cPanel.
Host lookup did not complete The server did not find the MX record for the recipient's address.
JunkMail rejected - (example.com) is in an RBL: Blocked The sender's domain has been found in a blacklist and the message was rejected.
No Such User Here / No such person at this address The recipient was not found on the server. This error occurred because you have incorrectly typed the recipient's address or because the user does not have the necessary permissions in the mail storage folder.
Rejected relay attempt: From: 'test@example.com' To: 'test@example.gr' The sender has not been authenticated correctly with the mail server or the recipient's server considered the sender's server to be a source of unwanted messages (spam).
Retry timeout exceeded The mail retry limit has been reached. This error can occur for a variety of reasons, such as that the recipient's mailbox or server disk space is full, the mail may may have been blocked by a spam filter or even by wrong access permissions in the recipient's folder.
Connection timed out The mail server is not available. This error occurs when a door (usually the SMTP port 25) is blocked by a firewall.
Domain example.com has an outgoing mail suspension. Message discarded The sender's domain can not send mail and the mails are rejected. This error typically occurs when a cPanel account has been moved from one server to another.
SMTP AUTH is required for message submission on port 587 The sender must be authenticated by the mail server before it can send an email.

Gmail examples

MessageExplanation
421, "4.4.5", Server busy, try again later. The server is busy try again later.
421, "4.7.0", IP not in whitelist for RCPT domain, closing connection. The IP address is not in the recipient's authorized IP list, the connection will be closed.
421, "4.7.0", Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily blocked. The system detected a large percentage of unwanted messages from your IP address and to protect it's users you have been temporarily blocked.
421, "4.7.0", Temporary System Problem. Try again later. Temporary system error occurred, please try again later.
450, "4.2.1" The user you are trying to contact is receiving mail too quickly. Please resend your message at a later time. If the user is able to receive mail at that time, your message will be delivered. The user you are trying to reach is receiving multiple messages at the same time. Send your message later, and it might be delivered.
450, "4.2.1", The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. Please resend your message at a later time. If the user is able to receive mail at that time, your message will be delivered. The user you are trying to communicate receives messages at a rate that prevents them from delivering. Send your message later,
and it might be delivered.
451, "4.3.0", Mail server temporarily rejected message. The message was temporarily rejected by the server.
451, "4.3.0", Multiple destination domains per transaction is unsupported. Please try again. Multiple destination addresses are not allowed. Try again.
451, "4.5.0", SMTP protocol violation, see RFC 2821. SMTP protocol violation.
452, "4.5.3", Your message has too many recipients. Your message has too many recipients.
454, "4.7.0", Cannot authenticate due to temporary system problem. Try again later. Unable to verify user due to system problems. Please try again later.
504, "5.7.4", Unrecognized Authentication Type. Unknown type of authentication.
530, "5.5.1", Authentication Required. Authentication is required.
535, "5.7.1", Username and Password not accepted. Username and password might be wrong.
550, "5.1.1", The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces. The email account you are trying to contact does not exist. Please check the recipient's email address for errors.
550, "5.2.1", The email account that you tried to reach is disabled. The email account you are trying to contact is disabled.
550, "5.4.5", Daily sending quota exceeded. You have exceeded the daily limit for sending messages.
552, "5.2.3", Your message exceeded Google's message size limits. Your message has exceeded Google's size limit.
553, "5.1.2", We weren't able to find the recipient domain. Please check for any spelling errors, and make sure you didn't enter any spaces, periods, or other punctuation after the recipient's email address. Could not find the recipient's domain. Check the recipient's email address for errors and try again.
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