Have you ever tried to send an email but received an error code that was completely foreign to you? These codes are actually crucial for understanding what’s wrong with your email.
If you encountered an error, there may be an issue with your Simple Mail Transfer Protocol (SMTP). SMTP ensures the overall functionality of email communication, enabling you to send and receive messages.
Here is the ultimate directory of SMTP error codes and messages and how to resolve them.
1XX - Informational responses
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Error Code
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Meaning
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The server has received the request and is proceeding to processing it.
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110
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The server has started a connection to the client and is waiting for the client to send a command.
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120
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The server is ready to transmit data
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2XX - Success messages
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Error Code and Message
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Meaning
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200: "OK"
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A general acknowledgment that the server has received and processed the request
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220: "Service ready."
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The server is prepared to start the conversation.
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221: "Service closing transmission channel"
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The server is ending the conversation, usually after successfully sending the email.
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250: "Requested mail action okay, completed."
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The action was successfully completed, and the session is progressing smoothly.
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Here is Warmy’s complete guide to SMTP Success Codes: 200, 220, 221, and 250.
3XX - Persistent transient negative completion replies
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Error Code and Message
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Meaning
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The server is ready to accept the email content but requires proper termination with [ CRLF] . [CRLF] (carriage return, line feed, period, carriage return, line feed)
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4XX - Temporary failures
The next batch of SMTP error codes and messages range from SMTP Error 421 to SMTP Error and their variants:
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Error Code and Message
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Meaning and sub-variants
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The server is temporarily unavailable. This could be due to heavy load or maintenance. The connection will be closed. Variants and respective fixes include:
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This error pertains to issues within the email content, which can be found in the email body, headers, or attachments.
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431 usually means that there is a momentary problem with the recipient’s mail server, such not enough system storage to handle your mail.
432 means recipient’s mailbox is full and the email could not be delivered. |
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This error is frequently seen when the server is unable to connect to the recipient’s email server for a variety of network or configuration reasons.
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This message alerts the sender that the email was not delivered, but the reason implies a potential resolution upon retrying or after some network adjustments
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These are particular problems that can impede email delivery that is intended to be successful. Every error code gives information on many kinds of issues with the email transmission.
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This is essentially a hiccup in the email delivery process, indicating a temporary error rather than a complete halt.
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The following table outlines SMTP Error 451 and its variants which refers to a temporary error in the email delivery process. It usually points to a local processing error on the receiving server.
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Error Code and Message
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Meaning
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The mail server of the recipient encountered an internal processing error preventing it from accepting or queueing your message at that moment. Variants include:
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Next, we go over SMTP Errors 452 to 471 and their respective variants.
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Error Code and Message
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Meaning
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The email server is temporarily overloaded or when the recipient’s mailbox has reached its quota. variants include:
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This occurs when an email sender tries to send messages to too many people in a short amount of period
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The server hits a snag in confirming your identity and needs a bit more to proceed with delivery. There are a couple of variants:
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These errors pertain to sending failure due to server overload or issues with server configuration.
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It is often related to server overload, maintenance, or other temporary conditions affecting the server’s ability to process emails.
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5XX - Permanent failure (non-retryable errors)
SMTP 5XX errors represent a permanent server-side failure. These issues don’t typically resolve on their own.
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Error Code and Message
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Meaning
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This error is less specific than others, not pinpointing exactly what tripped up the email’s journey. It’s a catch-all for when the email server faces unexpected issues, impacting both the sender’s ability to dispatch emails and the recipient’s chance of receiving them promptly.
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It generally occurs due to an incorrectly formatted email address or SMTP command lines when trying to send an email.
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This is a typical error that users may experience as an email client is unable to run a given command for the lack of recognition or ability for the server to implement it.
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The 503 error usually occurs when the email server requires the sender to authenticate their email address before sending an email.
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This error indicates a serious breakdown in communication between the servers involved in the email transmission process, not just a small annoyance.
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This specific issue encountered during email transmission that typically indicates a problem with the sender’s email address or its authentication.
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This pertains to a network or routing problem that occurs when the destination is unreachable due to DNS problems or server misconfiguration.
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These three errors pertain to issues regarding the recipient’s email addressor the email characteristics of the sender.
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This usually indicates that the server is not available at this time. This can be the result of maintenance, a server outage, or a permanent server shutdown.
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This error is most commonly traced back to an authentication issue
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This issue frequently appears as a notice indicating that the authentication attempt was unsuccessful, preventing the email from being delivered successfully.
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This is a specific response code indicating an authentication issue during the email sending process
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This is not a standard error and it might appear during the authentication phase of an email session, particularly when the server requires a higher level of security for the connection which the client does not support.
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The recipient’s email server refuses a message sent from your server.
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These are significant disruptions that can arise during the email sending and receiving process
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SMTP 550 to 555 errors and its variants pertain to issues with email content, potential spam, delivery routing, IP reputation, or misconfiguration. Let’s break them down:
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Error Code and Message
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Meaning
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This error message informs you that the email you attempted to send was unable to reach its intended recipient due to being identified as spam. Its variants are as follows:
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This is a permanent delivery failure, usually meaning the email was rejected due to a full recipient mailbox, an oversized email/attachment, or the message violating the recipient's policy (spam, malware, content).
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This error signifies that the email failed to send due to issues related to the issues surrounding the sender’s account, such as email address, IP reputation, or server configuration
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This response means your email was rejected by the receiving server. Variants include:
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This error can indicate a syntax issue in the parameters or arguments passed to the SMTP instructions, implying a misconfiguration or wrong format in the email sending request.
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Turning SMTP errors into actionable insights: Strengthen deliverability with Warmy
Knowing what the error message pertains to can help you understand the actions you can take to resolve it.
In most cases, receiving an SMTP error message may have something to do with the reputation of your email domain, to ensure that this is not the case, then you can utilize Warmy.io and its free email deliverability test.
Along with its email deliverability test, Warmy.io also wants to ensure that you can have a reputable email domain with SPF and DMARC generators that will provide a positive standing with different Internet Service Providers (ISP).
Warmy.io helps you stay ahead of SMTP errors by improving your sender reputation and ensuring your emails consistently reach the inbox.
With AI-powered warmup, real-time deliverability monitoring, DNS authentication tools, and inbox placement testing, Warmy gives you full visibility and control over your email performance.
Instead of guessing why emails bounce or get rejected, you’ll have clear insights and the tools to fix issues before they impact your campaigns.
Don’t let SMTP errors slow you down. Try Warmy for free today and build a stronger, more reliable email sending foundation.
