Email error codes are used by mail servers to indicate delivery status and hence help in diagnosing and resolving issues related to email delivery.
These codes can be categorized into different types based on their first digit:
2xx for successful delivery
3xx for further information needed
4xx for temporary failures
5xx for permanent failures
Here is a list of common email error codes and their meanings:
2xx — Successful Delivery
200: OK - A general acknowledgment that the server has received and processed the request.
211: System status or system help reply - Provides detailed server information for the user.
214: Help message - Offers information to help the user understand the commands.
220: Service ready - Indicates the server is ready to receive the next command.
221: Service closing transmission channel - Marks the end of the email delivery session.
235: Authentication successful - Shows the SMTP authentication test was passed.
250: Mail accepted by receiving network: Requested mail action completed - Indicates the email was successfully delivered.
251: User not local; message will be forwarded - The server will forward the email to another server.
252: Cannot verify user, but will accept message and attempt delivery - The server accepts the message even if it cannot verify the user.
253: Pending messages for node started - Informs the client that the server has started delivering pending messages to a particular node.
3xx — Further Information Needed
The most commonly used codes in the 3xx category are 334 and 354. Other codes in this range are not typically used in standard SMTP responses.
300-333: Not a standard SMTP code; typically, SMTP servers do not use this code.
334: Response to the AUTH command when the requested security mechanism is accepted. It provides the required SMTP authentication credentials to activate the next command. Once passed, it returns the 235 OK response.
340-353: Not a standard SMTP code; typically, SMTP servers do not use this code.
354: Start mail input; end with <CR><LF>.<CR><LF>. This indicates the server is ready to receive the message body.
360-384: Not a standard SMTP code; typically, SMTP servers do not use this code.
4xx — Temporary Failures
These codes indicate that the email delivery failed temporarily and may succeed if retried later. However, some of these codes (like 400 to 406) are less commonly used in standard SMTP error reporting for temporary failures. The most commonly referenced codes for temporary failures are 421, 450, 451, and 452.
400: Syntax error, command unrecognized.
401: Syntax error in parameters or arguments.
402: Command not implemented.
403: Command parameter not implemented.
404: Bad sequence of commands.
405: Command not allowed.
406: SMTP service not available.
420: Recipient's firewall/filter blocking mail
421: Domain service not available, closing transmission channel
422: Recipient has exceeded their mailbox limit / mailbox is over quota.
423: Mailbox busy.
424: Mailbox unavailable.
425: Mailbox transmission channel busy.
426: Mailbox unavailable.
427: Sender's address blocked.
428: SMTP service not available.
429: Too many messages.
430: Mailbox temporarily suspended.
431: Insufficient storage or out-of-memory issues / recipient's server is temporarily unavailable.
432: The recipient's server is not accepting messages at this time.
441: Intermittent network connection, the server has not yet responded
442: The server started to deliver the message but then the connection was broken
446: Too many hops. Most likely, the message is looping
447: Timeout occurred, check server connection
449: A DNS problem occurred; check your smart host setting on the SMTP connector
450: Requested action not taken; mailbox unavailable.
451: Requested action aborted: local error in processing / Temporary server error; try again later.
452: Requested action not taken: insufficient system storage
453: No mail.
454: Temporary authentication failure.
455: Recipient's SMTP server cannot interpret parameters associated with the "MAIL FROM" or "RCPT TO" commands
471: Anti-spam filter or firewall prevented message from sending
5xx — Permanent Failures
These codes provide more detailed information about the delivery failure:
500: Address does not exist
501: Syntax error in parameters or arguments
502: Command not implemented
503: Bad sequence of commands
504: Command parameter not implemented
510: Check the recipient address / Other address status
511: Bad destination mailbox address / Mailbox not found
512: Bad destination system address / Domain can not be found. Unknown host
513: Bad destination mailbox address syntax / Relaying denied or Address type incorrect
514: Destination mailbox address ambiguous
515: Destination mailbox address invalid
515: Destination mailbox address valid
516: Mailbox has moved
517: Bad sender’s mailbox address syntax / Problem with sender's mail attribute, check properties
518: Bad sender’s system address
520: Other or undefined mailbox status
521: Domain does not accept mail / Mailbox disabled, not accepting messages
522: Mailbox full / Recipient has exceeded mailbox limit
523: Message length exceeds administrative limit / Server limit exceeded. Message too large
524: Mailing list expansion problem
530: Access Denied. Authentication required / Other or undefined mail system status
531: Mail system Full
532: System not accepting network messages
533: Remote server has insufficient disk space to hold email / System not capable of selected features
534: Authentication mechanism is too weak. Message too big / Message too big for system
535: Multiple servers using same IP. Required Authentication
538: Encryption required for requested authentication mechanism
540: Email address has no DNS Server / Other or undefined network or routing status
541: No answer from host / No response from host
542: Bad Connection
543: Routing server failure. No available route
544: Unable to route
545: Network congestion
546: Email looping / Routing loop detected
547: Delivery time-out / Delivery time expired
550: Requested action not taken: mailbox unavailable / Other or undefined protocol status
551: Invalid command / User not local; please try forward path
552: Requested mail action aborted: exceeded storage allocation / Syntax error
553: Requested action not taken: mailbox name not allowed / Too many recipients
554: Invalid command arguments / Transaction failed
555: Wrong protocol
556: Email sent is too large for the recipient's email server to process
