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Error code wiki · 105 codes

The standard SMTP codes

The reply codes of the SMTP standard (RFC 5321 and its updates) and the enhanced status codes kept by IANA, which any mail server can send, whoever runs it.

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Permanent failures (5xx)

85 codes

The receiving server refused the message for good. Your server will not try again, and the message comes back to you as a bounce.

  1. 5.0.0Unspecified failure.The server reports a failure with no detail beyond its class: 5.0.0 means the message was refused for good, 4.0.0 means a temporary problem that will be retried.Permanent
  2. 5.1.0Address problem, unspecified.Something about an address in the message, the sender's or a recipient's, made the server refuse it, without saying exactly what was wrong.Permanent
  3. 5.1.1Mailbox does not exist.The recipient's domain accepts mail, but the mailbox named before the @ does not exist there, so the message was refused and will not be delivered.Permanent
  4. 5.1.2Recipient domain cannot receive mail.The part of the address after the @ names a domain that does not exist or cannot accept mail, so the message cannot be delivered there.Permanent
  5. 5.1.3Recipient address is malformed.The recipient address breaks the rules for how an email address must be written, so the server refused it before checking whether it exists.Permanent
  6. 5.1.4Recipient address is ambiguous.The recipient address matches more than one mailbox on the receiving system, so the server could not decide which person should get the message.Permanent
  7. 5.1.6Mailbox has moved.The address used to be valid, but the mailbox no longer takes mail and the server has no new address to forward the message to.Permanent
  8. 5.1.7Sender address is malformed.The sender address in the envelope (MAIL FROM) is written in a way the receiving server cannot accept, so it refused the message.Permanent
  9. 5.1.8Sender domain cannot receive mail.The domain in the sender address does not exist or cannot accept mail, so the receiving server refuses a message whose bounces could never be returned.Permanent
  10. 5.2.0Mailbox problem, unspecified.The recipient's mailbox exists, but something about it stopped the delivery and the receiving server did not say what.Permanent
  11. 5.2.1Mailbox disabled.The recipient's mailbox exists but has been switched off and is refusing messages, for a while or for good depending on the class of the code.Permanent
  12. 5.2.2Mailbox full.The recipient's mailbox has reached its storage quota, so the server will not take more mail until the owner deletes messages or gets more space.Permanent
  13. 5.2.3Message too large for this mailbox.The message is bigger than the size limit set for this particular mailbox, which an administrator set lower than the system-wide limit.Permanent
  14. 5.2.4Mailing list could not be expanded.The address is a mailing list, and the server could not turn it into the list of member addresses, so the message did not go out to the list.Permanent
  15. 5.3.0Receiving system problem, unspecified.The receiving server normally accepts mail, but a problem on that system itself, rather than with the mailbox or the message, stopped this delivery.Permanent
  16. 5.3.2Server not accepting mail.The server holding the mailbox is not accepting messages at all, because it is shutting down, overloaded, in maintenance, or set never to receive mail.Permanent
  17. 5.3.3Requested feature not supported.The message relies on a feature that the receiving system cannot provide, typically at a gateway that cannot carry it into another mail environment.Permanent
  18. 5.3.4Message too big.The message is larger than the maximum size the receiving system accepts for any message, so it was refused and has to be made smaller.Permanent
  19. 5.3.5Receiving system misconfigured.The receiving system's own configuration does not allow it to accept this message, a problem only its administrator can fix.Permanent
  20. 5.4.4No route to the recipient domain.The mail system could not work out where to send the message next, because DNS gave no usable mail server for the recipient domain.Permanent
  21. 5.4.6Mail routing loop.The message was passed back and forth between servers, or forwarded around a circle, until a server noticed the loop and stopped it.Permanent
  22. 5.5.0SMTP protocol problem, unspecified.Something went wrong in the SMTP conversation between the servers that the receiving side did not describe with a more specific protocol code.Permanent
  23. 5.5.1Invalid SMTP command.The sending software issued an SMTP command that the server does not support, or sent it at the wrong point in the conversation.Permanent
  24. 5.5.2SMTP syntax error.A command from the sending software could not be understood, because its syntax was wrong or the command was not recognised at all.Permanent
  25. 5.5.4Invalid command arguments.A valid SMTP command was sent with an argument the server does not accept, because the value was out of range or named a feature it does not know.Permanent
  26. 5.5.5Protocol version mismatch.The two mail systems could not agree on a version of the delivery protocol, and they could not work around the difference on their own.Permanent
  27. 5.6.0Message content problem, unspecified.Something about the content of the message made it undeliverable, and the server did not say exactly what.Permanent
  28. 5.6.1Content type not supported.The message uses a kind of content that the delivery protocol or the next system on the path cannot carry, so it was refused.Permanent
  29. 5.6.2Conversion needed but not allowed.The content would have to be converted to be delivered, but the message itself or the sending host's policy forbids that conversion.Permanent
  30. 5.6.3Conversion needed but not possible.The content has to be converted before it can be passed on, but a server on the path is not able to perform that conversion.Permanent
  31. 5.6.5Conversion failed.A server tried to convert the message content so it could be delivered, and the conversion did not succeed.Permanent
  32. 5.6.6Message content not available.During submission, the server had to fetch the message content from another system, such as an IMAP server, and could not get it.Permanent
  33. 5.6.7Non-ASCII address not allowed.An address in the envelope contains non-ASCII characters (an internationalised address), and the receiving server will not accept that for this sender or recipient.Permanent
  34. 5.6.8UTF-8 reply needed but not allowed.To answer a VRFY or EXPN query the server would have to show a mailbox name in UTF-8, but the client did not say it can handle UTF-8 replies.Permanent
  35. 5.6.9Internationalised message cannot be delivered.The message has UTF-8 (internationalised) headers and could not be passed on to one or more recipients whose path cannot handle them, so it was rejected.Permanent
  36. 5.6.10Deprecated duplicate of 5.6.8.A registered code that duplicates X.6.8 (UTF-8 reply required but not permitted) and is deprecated, so servers should send X.6.8 instead.Permanent
  37. 5.7.0Security or policy refusal, unspecified.The message or session was refused for a security or policy reason that the server did not describe more precisely, or chose not to reveal.Permanent
  38. 5.7.1Delivery not authorised.The receiving server refuses the message because this sender is not allowed to send to this recipient, as a result of filtering by host or by recipient.Permanent
  39. 5.7.2Not allowed to send to this list.The sender is not authorised to post to the mailing list it wrote to, so the list refused the message.Permanent
  40. 5.7.3Secure format conversion not possible.Delivery needed the message converted from one secure messaging format to another, and that conversion could not be done.Permanent
  41. 5.7.4Security feature not supported.The message or session asked for a security feature, such as a form of secure authentication, that the delivery protocol here cannot support.Permanent
  42. 5.7.5Cryptographic failure.A system that was authorised to check or decrypt the message could not do it, because a key or other required information was missing or invalid.Permanent
  43. 5.7.6Cryptographic algorithm not supported.A system that should validate or decrypt the message could not, because it does not support the algorithm the message uses.Permanent
  44. 5.7.7Message integrity check failed.A system authorised to validate the message found it corrupted or altered, so the check on its integrity failed.Permanent
  45. 5.7.10Encryption needed.The server requires an encrypted connection (TLS) before this kind of login can happen, or a message that demands TLS could not be passed on securely.Permanent
  46. 5.7.13User account disabled.The login itself worked, but the account has been disabled by its administrator, so the server will not accept mail from it until it is re-enabled.Permanent
  47. 5.7.14Trust relationship required.The submission server needs a configured trust relationship with another server to fetch the message content, and none is in place.Permanent
  48. 5.7.15Priority too low.The server only accepts messages above a certain transfer priority at the moment, and the priority of this message is below that level.Permanent
  49. 5.7.17Mailbox owner has changed.The message asked to be delivered only if the recipient address had the same owner since a given date, and the receiving system says it has changed hands.Permanent
  50. 5.7.18Domain owner has changed.Under an RRVS check, the receiving system reports that the recipient's domain has changed owner since the date the sender gave, so the message was refused.Permanent
  51. 5.7.19Ownership check could not be done.The message asked the receiver to check how long the recipient had owned the mailbox, and the receiver could not, because it has no record of that date.Permanent
  52. 5.7.24SPF check error.The receiving server could not complete the SPF check for the sender's domain, either because of a temporary DNS problem or because the SPF record is broken.Permanent
  53. 5.7.28Mail flood detected.The receiving system thinks the message is part of a flood of similar abusive messages, such as one caused by forged web form signups, and deferred or rejected it.Permanent
  54. 5.7.29ARC validation failed.The message carried an ARC chain, the record that forwarders add when they pass mail on, and the receiver found that chain invalid.Permanent
  55. 5.7.30REQUIRETLS not supported by the next server.The message was sent with a REQUIRETLS requirement, and none of the servers it had to go through next supports that requirement, so it was returned.Permanent
  56. 500Command not recognised.The server did not recognise the command it received, or the command line was too long. This is a permanent error in the SMTP conversation.Permanent
  57. 500 5.5.6Authentication data too long.During login, the client sent an authentication response longer than the server can accept for the chosen method, so the AUTH command failed.Permanent
  58. 501Syntax error in arguments.The command was recognised, but what came with it was malformed, often an address or a host name the server cannot accept.Permanent
  59. 502Command not implemented.The server recognises the command but does not implement it, so it will not carry that command out in this session.Permanent
  60. 503Bad sequence of commands.The client sent a command at the wrong point in the conversation, for example message data before any recipient was accepted, or mail before logging in.Permanent
  61. 504Command parameter not implemented.The server implements the command, but not the particular parameter or option the client asked for with it.Permanent
  62. 521Host does not accept mail.The server answered the connection only to say that it never accepts mail, so delivery to this host cannot work.Permanent
  63. 530Authentication required.The server requires the client to log in, or to start TLS, before it accepts this command, which usually means before it accepts any mail.Permanent
  64. 534Authentication method too weak.The server refused the login because the authentication method the client chose is weaker than its policy allows for this user.Permanent
  65. 534 5.7.9Authentication method too weak.The server refused the login because the chosen authentication method is weaker than its policy allows for this user; the client should pick a stronger one.Permanent
  66. 535Authentication credentials invalid.The server rejected the login because the username, password or token is wrong or not sufficient for this account, so no mail is sent.Permanent
  67. 535 5.7.8Login failed: invalid credentials.The SMTP server rejected the login because the username, password or token was wrong or not sufficient, so it will not accept mail from this session.Permanent
  68. 538Encryption required for this login method.The authentication method the client chose is only allowed over an encrypted connection, and this connection is not encrypted.Permanent
  69. 538 5.7.11Encryption required for this login method.The chosen authentication method may only be used over an encrypted connection, and the current SMTP connection is not encrypted.Permanent
  70. 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.Permanent
  71. 550 5.7.20No valid DKIM signature.The receiving server requires a valid DKIM signature and found none that passed verification, so it refused the message.Permanent
  72. 550 5.7.21No acceptable DKIM signature.The message has at least one DKIM signature that verifies, but none meets the receiving server's own requirements, so it was refused.Permanent
  73. 550 5.7.22No DKIM signature from the From domain.The message has DKIM signatures that verify, but none is from the domain in the From header, which the receiving server requires.Permanent
  74. 550 5.7.23SPF check failed.The SPF check on the sending server's IP address gave a fail result for the sender's domain, and the receiving server's policy rejects such mail.Permanent
  75. 550 5.7.25Reverse DNS check failed.The sending server's IP address failed the receiver's reverse DNS check, usually because it has no PTR record or the PTR name does not lead back to the IP.Permanent
  76. 550 5.7.26Multiple authentication checks failed.The message failed more than one authentication check, for example SPF and DKIM, and the receiving server's policy rejects it.Permanent
  77. 550 5.7.27Sender domain has a null MX.The domain in the sender address publishes a null MX record, which says it never receives mail, so the receiver refuses mail whose bounces could not be delivered.Permanent
  78. 551User not local.The server does not deliver mail for this recipient and may suggest another address to try. The message was not accepted.Permanent
  79. 552Storage allocation exceeded.The receiving server aborted the message because it exceeded a storage limit, usually a full mailbox or a message larger than allowed.Permanent
  80. 552 5.7.16Too big for its priority.The server limits message size per priority level, and this message is larger than allowed for the priority it was sent with.Permanent
  81. 553Mailbox name not allowed.The server refused the address because the mailbox name is not allowed there, most often because it is written incorrectly.Permanent
  82. 554Transaction failed.The server permanently refused the transaction, often after receiving the message data or straight away when the connection opened.Permanent
  83. 555Parameters not recognised.The server does not recognise, or does not implement, a parameter given with MAIL FROM or RCPT TO, so that command failed.Permanent
  84. 556Domain does not accept mail.A relay or sending server knows the recipient's domain does not accept mail, usually from its null MX record, and refused the address without trying to deliver.Permanent
  85. 556 5.1.10Recipient domain has a null MX.The recipient's domain publishes a null MX record, its formal statement that it never receives email, so the message was refused at once without any delivery attempt.Permanent

Temporary failures (4xx)

16 codes

The receiving server cannot take the message now. Your server keeps it and retries, and only bounces it if the retries run out.

  1. 4.3.1Receiving system out of storage.The receiving mail server has run out of storage space, so it cannot accept messages for now; the sending server keeps the message and retries later.Temporary
  2. 4.4.0Network or routing problem, unspecified.Something went wrong with the network or the routing between mail servers, without a more specific explanation, and delivery is usually retried.Temporary
  3. 4.4.1No answer from the receiving server.The sending server tried to connect to the receiving server and got no answer, so it keeps the message and tries again later.Temporary
  4. 4.4.2Connection dropped.The connection to the receiving server was opened, but broke or timed out before the message transfer finished; the sending server will try again.Temporary
  5. 4.4.3DNS or directory lookup failed.The message could not be passed on because a directory server, most often DNS, did not answer; the lookup and the delivery are retried.Temporary
  6. 4.4.5Mail system congested.The mail system was too busy to deliver the message at that moment, so delivery is delayed and will be tried again.Temporary
  7. 4.4.7Delivery time expired.The message stayed undelivered for too long, usually because every retry failed, so the server gave up on it or warned that it is delayed.Temporary
  8. 4.5.3Too many recipients.The message lists more recipients than the server accepts in one transaction; the remaining recipients have to be sent in a separate transaction.Temporary
  9. 421Service not available.The receiving server is closing the connection and will not take mail right now. The reply is temporary, so the sending server keeps the message and tries again later.Temporary
  10. 432Password transition needed.A temporary reply to a login attempt: the user has to move to the authentication method that was requested, which is usually done by logging in once with the PLAIN method.Temporary
  11. 432 4.7.12Password transition needed.A temporary login reply: the user has to move to the authentication method that was requested, which is usually done by logging in once with the PLAIN method.Temporary
  12. 450Mailbox temporarily unavailable.The server did not take the message for this mailbox for now, because the mailbox is busy or temporarily blocked by policy. The sending server will try again.Temporary
  13. 451Local error in processing.The receiving server hit a problem on its own side while handling the message and stopped. The reply is temporary, so the sending server will try again.Temporary
  14. 452Insufficient system storage.The receiving server is short of storage or has hit a limit, such as the number of recipients in one message, and asks the sender to try again later.Temporary
  15. 454Temporary authentication or TLS failure.A login or a TLS handshake could not be completed because of a temporary problem on the server; the client should try again later without changing anything.Temporary
  16. 455Parameters cannot be handled now.The server cannot handle one of the parameters given with MAIL FROM or RCPT TO at the moment, and asks the sender to try again later.Temporary

Success (2xx)

4 codes

The server accepted the command or the message. These appear in logs, not in bounces.

  1. 2.1.5Recipient address accepted.A success code: the receiving server checked the recipient address and accepts it. It appears in normal SMTP replies and positive delivery reports, never as an error.Success
  2. 2.1.9Relayed to a system without tracking.A message tracking status: the address was valid and the message was passed on to a system that does not support tracking, so no later status is available.Success
  3. 2.3.6Message priority changed.A success code from the MT-PRIORITY extension: the server accepted the message but did not keep the transfer priority the sender asked for.Success
  4. 2.6.4Delivered after a lossy conversion.Usually a warning on a successful delivery: the message arrived, but a conversion on the way lost some of its content.Success

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