SMTP Error Codes and Their Meaning

When an email cannot be delivered successfully, the sending mail server typically returns an SMTP error code. These codes help identify the reason for the delivery failure and can be useful when troubleshooting email issues.

SMTP error codes usually consist of three digits, sometimes followed by an extended status code such as:

554 5.7.1

Contents

• Understanding SMTP Status Codes
• Common 554 5.7.1 Errors
• SMTP Error Code Reference

Understanding SMTP Status Codes

First Digit

The first digit indicates the overall result of the command.

Code	Meaning
1xx	Command accepted, but additional confirmation is required (rarely used)
2xx	Success – the requested action completed successfully
3xx	Additional information is required
4xx	Temporary failure – retrying later may succeed
5xx	Permanent failure – the action could not be completed

Second Digit

The second digit provides additional information about the category of the response.

Code	Meaning
x0x	Syntax-related status
x1x	Informational response
x2x	Connection status
x3x	Reserved
x4x	Reserved
x5x	Mail system status

Third Digit

The third digit provides more specific information about the error condition.

Common 554 5.7.1 Errors

The status code:

554 5.7.1

is one of the most frequently encountered SMTP delivery errors and generally indicates a permissions, authentication, or anti-spam issue.

Error Message	Explanation
Delivery not authorized	Message delivery was not authorized.
You are not allowed to connect	Connection to the server was refused.
Client host rejected: Access denied	The sending host has been blocked.
Service unavailable; Client host blocked	The sender’s server has been denied access.
Relay access denied	Relaying through the server is not permitted.
Recipient address rejected: Access denied	The recipient address was rejected.
Message cannot be accepted, rules rejection	Server rules prevented delivery.
Message refused by Dictionary check	Message was rejected by content filtering.
Relay Access Denied for One Domain	Relaying is not allowed for the specified domain.
Message cannot be accepted, spam rejection	Message was identified as possible spam.
Message refused by DNSBL check	Sending IP address appears on a blacklist.
Sender address rejected: Access denied	Sender address was not authorized.

SMTP Error Code Reference

Success and Informational Codes

Code	Description
211	System status message
214	Help message
220	SMTP service ready
221	SMTP service closing connection
250	Requested action completed successfully
251	Recipient is not local, message will be forwarded
252	Recipient cannot be verified, delivery will be attempted
354	Server ready to receive message content

Temporary Failures (4xx)

Code	Description
421	Service unavailable, connection closed
422	Mailbox quota exceeded
431	Insufficient storage space
432	Delivery queue temporarily frozen
441	Network connection interrupted
442	Connection lost during delivery
446	Too many delivery hops detected
447	Connection timeout
449	DNS configuration issue
450	Mailbox unavailable
451	Temporary server error
452	Insufficient server resources
465	Language or character encoding issue

Permanent Failures (5xx)

Code	Description
500	Syntax error, command not recognized
501	Syntax error in command parameters
502	Command not implemented
503	Bad command sequence
504	Command parameter not implemented

Address and Recipient Errors (51x)

Code	Description
510	Invalid recipient address
511	Recipient address problem
512	Hostname not found
513	Missing or invalid address field
514	Duplicate address conflict
515	Invalid destination mailbox
516	Mailbox may have been moved
517	Sender address problem

Message Size Errors (52x)

Code	Description
521	Message too large
522	Recipient mailbox quota exceeded
523	Message exceeds recipient size limits
524	Distribution list processing problem

Mail System Errors (53x)

Code	Description
530	Mail transport service issue
531	Mail system storage full
532	System temporarily not accepting mail
533	Insufficient disk space on destination server
534	Message too large
535	Mail routing loop detected

Routing and DNS Errors (54x)

Code	Description
540	DNS resolution problem
541	No response from destination host
542	Connection problem
543	Routing failure
544	Next routing hop unavailable
546	Mail loop detected
547	Delivery timeout
548	Recipient policy restriction

Mailbox and Delivery Errors (55x)

Code	Description
550	Mailbox unavailable or message rejected
551	Recipient not local; forwarding address provided
552	Storage allocation exceeded
553	Invalid mailbox name or too many recipients
554	Transaction failed
555	Unsupported protocol version

Security and Policy Errors (56x–57x)

Code	Description
563	Too many attachments
571	Permission or access problem
572	Distribution list unavailable
573	Internal IP-related error
574	Security feature not supported
575	Cryptographic error
576	Certificate validation issue
577	Message integrity error

Troubleshooting Tips

Before investigating a specific SMTP error, verify the following:

• The sender’s email account credentials are correct.
• SMTP authentication is enabled.
• The domain’s email service is active.
• DNS records (MX, SPF, DKIM, and DMARC) are configured correctly.
• The sending server’s IP address is not listed on email blacklists.
• The recipient’s mailbox exists and has available storage space.

Summary

SMTP error codes provide valuable information about why an email could not be delivered. Understanding the meaning of the code returned by the mail server can help identify issues related to:

• Authentication
• Mailbox availability
• DNS configuration
• Spam filtering
• Message size limits
• Server connectivity

When troubleshooting delivery failures, always start by examining the complete SMTP error message, as it usually contains more detailed information than the numeric code alone.