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Most common mail error codes explained

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    JUMP TO MAIL ERROR CODES

    This article describes the various SMTP (outgoing) error codes that senders may receive when communicating with the Outgoing mail service.

    Kindly note:
    Not all reply codes are error codes. Sometimes it is simply a response, containing info about the server or an answer to a specific command.

    Simple explanation of both 400-and-500 SMTP error codes:

    400 SMTP error codes  –  Remote server did not accept delivery of e-Mail message.  This error condition is only temporary and the e-Mail message may be resent.
    500 SMTP error codes  –  The e-Mail message could not be delivered due a terminal error, even though some “permanent” errors can be corrected, e.g. Please ensure that the e-Mail address is complete and correct.

    Quite often, an e-Mail message could not be delivered due to being labeled as spam or simply because the sender and/or sender’s network has been listed as a probable source of spam.  Kindly visit MX TOOLBOX in order to check the health of your domain and/or IP address.

    Various 550 errors are returned once a mail server check the links to malware:

    • 550 (5.7.1): Service unavailable: client [###] blocked using [###]
    • 550 (5.7.1)Message rejected as spam by Content Filtering
    • 550 This message was classified as SPAM and may not be delivered
    • 550 High probability of spam (Gmail)
    • 550 (5.2.1) mail from [###] refused spam site
    • 550 Your message has been rejected because you have been detected sending spam
    • 550 Message contained unsafe content

    You can jump to the relevant error message by clicking on the appropriate code below:

    421 Service not available, closing transmission channel / connection
    451 Requested action aborted: error in processing
    451 Requested action aborted: This mail account has sent too many messages in a short amount of time. Please try again later.
    451 ESMTP MailEnable Service temporarily refused connection at [time] from IP (xxx.xxx.xxx.xxx) because of policy violation
    452 Too many recipients
    452 You are not permitted to send to remote domains
    501 Invalid Address
    503 This mail server requires authentication when attempting to send to a non-local e-mail address.  Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.
    550 Requested action not taken: mailbox unavailable or not local
    551 This mail server requires authentication before sending mail from a locally hosted domain. Please reconfigure your mail client to authenticate before sending mail.
    554 The IP address of the sender (xxx.xxx.xxx.xxx) was found in a DNS blacklist database and was therefore refused

    421 Service not available, closing transmission channel / connection

    Error Description:  The recipient’s e-Mail server or ISP (Internet Service Provider) is currently down.  This may occur during reboot of the mail server or any other maintenance/repair operation during which the mail service is temporarily suspended.

    451 Requested action aborted: error in processing

    Error Description:  You would generally receive this error message when the recipient’s ISP (Internet Service Provider) is overloaded due to a possible spam attack.  Please wait for a while before attempting to send another message.

    451 Requested action aborted: This mail account has sent too many messages in a short amount of time. Please try again later.

    Error Description:  This simply means that the SMTP configuration setting to limit the quantity of messages sent per hour (140) has been reached. The message sent in this instance has failed and will need to be retried by the client.

    451 ESMTP MailEnable Service temporarily refused connection at [time] from IP (xxx.xxx.xxx.xxx) because of policy violation

    Error Description: This response is sent by the SMTP (outgoing) server when the connecting IP address is registered in the internal IP abuse register. This register contains the IP addresses that have failed too many authentication attempts over a short period of time. IP addresses will be listed if they fail to authenticate over 10 times in less than an hour. When this happens the connection is blocked, but will be removed from the register after one (1) hour.

    452 Too many recipients

    Error Description:  Simply means that the SMTP (outgoing) configuration setting to limit the amount of inbound recipients per transaction or message has been exceeded.  This can be increased depending on the amount of employees in your company.

    452 You are not permitted to send to remote domains

    Error Description: When this error is received or reflects in the logs, it means that the internal MailEnable configuration setting, to limit a mailbox or server from sending to external domains, has been enabled. By default this setting is not enabled but when changed it can prevent a single mailbox, or even all mailboxes on the server to only be able to send locally to their own domain.

    The following error messages (500-504) usually tell you that your email client is broken or, most commonly, that your email could not be delivered for one reason or another.

    501 Invalid Address

    Error Description:  This error means that the address format used in the sending client is not correct. Usually this is checked in a standard retail client but when using scripts or web pages it may not be the case.

    Please note – always use lower case characters, numbers and the underscore to create your e-Mail address.

    503 This mail server requires authentication when attempting to send to a non-local e-mail address.  Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.

    Error Description: This error means that the server or client has connected to the MailEnable SMTP service but has not entered a valid RCPT TO mail address that exists on the server, although the domain for that address does exist on the server.  The order of checks in this situation are; 1. Does the domain exist on the server (Yes) 2. Does the address exist in the address-map.tab file or table on the server (No) 3. Has the client authenticated. (Due to earlier No at 2 this check has not occurred). As the connection has not authenticated, meaning that one of the options above in 1 or 2 have not been met then the error is returned.

    550 Requested action not taken: mailbox unavailable or not local

    Error Description: This error means that the server or client has connected to the MailEnable SMTP service but has not entered a RCPT TO mail address or domain that exists on the server.

    551 This mail server requires authentication before sending mail from a locally hosted domain. Please reconfigure your mail client to authenticate before sending mail.

    Error Description: This error means that a server or client has connected to the SMTP service to send email, using a sender address domain which exists on the mail server, but has not authenticated. For this to occur the Address Spoofing option has to be changed to prevent this. For example, if a client is sending with the FROM address of bob@example.com and the example.com domain exists and the connection has not authenticated, then this error will occur. This option is configured under the SMTP security options “Address Spoofing” located under the “Security” tab of the SMTP properties.

    The 551 error can also mean that the mailbox that has been entered into the MAIL FROM address line is local on the server but has been disabled on the server by a server administrator or in some cases a control panel.

    554 The IP address of the sender (xxx.xxx.xxx.xxx) was found in a DNS blacklist database and was therefore refused.

    Error Description: This error means that an inbound connection has been detected as being listed in one of your configured DNS blacklists within the SMTP options.  In short if the sender is blacklisted and you want to receive mail from this sender then you should suggest/request that they remove themselves from the blacklist.

    We will continue to add the most frequently received error messages to the above list.  If you stumble upon an error code not listed here, please view our advanced mail error codes page.

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