Internet Assigned Numbers Authority Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry Created 2008-05-29 Last Updated 2024-12-06 Available Formats [IMG] XML [IMG] HTML [IMG] Plain text Registries included below • Class Sub-Codes • Subject Sub-Codes • Enumerated Status Codes Class Sub-Codes Registration Procedure(s) Specification Required Expert(s) Chris Newman Reference [RFC5248] Available Formats [IMG] CSV Code Summary Description Reference Submitter Change Controller Success specifies that the DSN is reporting a positive [RFC3463] (Standards 2.XXX.YYY Success delivery action. Detail sub-codes may provide notification of track) G. Vaudreuil IESG transformations required for delivery. A persistent transient failure is one in which the message as Persistent Transient sent is valid, but persistence of some temporary condition [RFC3463] (Standards 4.XXX.YYY Failure has caused abandonment or delay of attempts to send the track) G. Vaudreuil IESG message. If this code accompanies a delivery failure report, sending in the future may be successful. A permanent failure is one which is not likely to be resolved 5.XXX.YYY Permanent Failure by resending the message in the current form. Some change to [RFC3463] (Standards G. Vaudreuil IESG the message or the destination must be made for successful track) delivery. Subject Sub-Codes Registration Procedure(s) Specification Required Expert(s) Chris Newman Reference [RFC5248] Available Formats [IMG] CSV Code Summary Description Reference Submitter Change Controller X.0.YYY Other or Undefined There is no additional subject information available. [RFC3463] (Standards G. Vaudreuil IESG Status track) The address status reports on the originator or destination X.1.YYY Addressing Status address. It may include address syntax or validity. These [RFC3463] (Standards G. Vaudreuil IESG errors can generally be corrected by the sender and track) retried. Mailbox status indicates that something having to do with [RFC3463] (Standards X.2.YYY Mailbox Status the mailbox has caused this DSN. Mailbox issues are assumed track) G. Vaudreuil IESG to be under the general control of the recipient. Mail system status indicates that something having to do X.3.YYY Mail System Status with the destination system has caused this DSN. System [RFC3463] (Standards G. Vaudreuil IESG issues are assumed to be under the general control of the track) destination system administrator. The networking or routing codes report status about the delivery system itself. These system components include any X.4.YYY Network and Routing necessary infrastructure such as directory and routing [RFC3463] (Standards G. Vaudreuil IESG Status services. Network issues are assumed to be under the track) control of the destination or intermediate system administrator. The mail delivery protocol status codes report failures X.5.YYY Mail Delivery Protocol involving the message delivery protocol. These failures [RFC3463] (Standards G. Vaudreuil IESG Status include the full range of problems resulting from track) implementation errors or an unreliable connection. The message content or media status codes report failures involving the content of the message. These codes report Message Content or Media failures due to translation, transcoding, or otherwise [RFC3463] (Standards X.6.YYY Status unsupported message media. Message content or media issues track) G. Vaudreuil IESG are under the control of both the sender and the receiver, both of which must support a common set of supported content-types. The security or policy status codes report failures involving policies such as per-recipient or per-host filtering and cryptographic operations. Security and policy X.7.YYY Security or Policy status issues are assumed to be under the control of either [RFC3463] (Standards G. Vaudreuil IESG Status or both the sender and recipient. Both the sender and track) recipient must permit the exchange of messages and arrange the exchange of necessary keys and certificates for cryptographic operations. Enumerated Status Codes Registration Procedure(s) Specification Required Expert(s) Chris Newman Reference [RFC5248] Available Formats [IMG] CSV Associated Code Sample Text basic status Description Reference Submitter Change Controller code Other undefined status is the only X.0.0 Other undefined Any undefined error code. It should be [RFC3463] (Standards Track) G. Vaudreuil IESG Status used for all errors for which only the class of the error is known. Other address Something about the address X.1.0 status Not given specified in the message caused this [RFC3463] (Standards Track) G. Vaudreuil IESG DSN. The mailbox specified in the address does not exist. For Internet mail X.1.1 Bad destination 451, 550 names, this means the address [RFC3463] (Standards Track) G. Vaudreuil IESG mailbox address portion to the left of the "@" sign is invalid. This code is only useful for permanent failures. The destination system specified in the address does not exist or is incapable of accepting mail. For X.1.2 Bad destination Not given Internet mail names, this means the [RFC3463] (Standards Track) G. Vaudreuil IESG system address address portion to the right of the "@" is invalid for mail. This code is only useful for permanent failures. The destination address was Bad destination syntactically invalid. This can X.1.3 mailbox address 501 apply to any field in the address. [RFC3463] (Standards Track) G. Vaudreuil IESG syntax This code is only useful for permanent failures. The mailbox address as specified matches one or more recipients on Destination mailbox the destination system. This may X.1.4 address ambiguous Not given result if a heuristic address [RFC3463] (Standards Track) G. Vaudreuil IESG mapping algorithm is used to map the specified address to a local mailbox name. This mailbox address as specified X.1.5 Destination address 250 was valid. This status code should [RFC3463] (Standards Track) G. Vaudreuil IESG valid be used for positive delivery reports. The mailbox address provided was at Destination mailbox one time valid, but mail is no X.1.6 has moved, No Not given longer being accepted for that [RFC3463] (Standards Track) G. Vaudreuil IESG forwarding address address. This code is only useful for permanent failures. Bad sender's The sender's address was X.1.7 mailbox address Not given syntactically invalid. This can [RFC3463] (Standards Track) G. Vaudreuil IESG syntax apply to any field in the address. The sender's system specified in the address does not exist or is X.1.8 Bad sender's system 451, 501 incapable of accepting return mail. [RFC3463] (Standards Track) G. Vaudreuil IESG address For domain names, this means the address portion to the right of the "@" is invalid for mail. The mailbox address specified was Message relayed to valid, but the message has been X.1.9 non-compliant Not given relayed to a system that does not [RFC3886] (Standards Track) E. Allman IESG mailer speak this protocol; no further information can be provided. Recipient address This status code is returned when [RFC7505] (Standards Track); J. Levine, X.1.10 has null MX 556 the associated address is marked as [RFC7504] (Standards Track) M. Delany, IESG invalid using a null MX. J. Klensin Other or undefined The mailbox exists, but something X.2.0 mailbox status Not given about the destination mailbox has [RFC3463] (Standards Track) G. Vaudreuil IESG caused the sending of this DSN. The mailbox exists, but is not Mailbox disabled, accepting messages. This may be a X.2.1 not accepting Not given permanent error if the mailbox will [RFC3463] (Standards Track) G. Vaudreuil IESG messages never be re-enabled or a transient error if the mailbox is only temporarily disabled. The mailbox is full because the user has exceeded a per-mailbox administrative quota or physical X.2.2 Mailbox full 552 capacity. The general semantics [RFC3463] (Standards Track) G. Vaudreuil IESG implies that the recipient can delete messages to make more space available. This code should be used as a persistent transient failure. A per-mailbox administrative message Message length length limit has been exceeded. This exceeds status code should be used when the X.2.3 administrative 552 per-mailbox message length limit is [RFC3463] (Standards Track) G. Vaudreuil IESG limit less than the general system limit. This code should be used as a permanent failure. The mailbox is a mailing list Mailing list address and the mailing list was X.2.4 expansion problem 450, 452 unable to be expanded. This code may [RFC3463] (Standards Track) G. Vaudreuil IESG represent a permanent failure or a persistent transient failure. 221, 250, The destination system exists and X.3.0 Other or undefined 421, 451, normally accepts mail, but something [RFC3463] (Standards Track) G. Vaudreuil IESG mail system status 550, 554 about the system has caused the generation of this DSN. Mail system storage has been exceeded. The general semantics imply that the individual recipient X.3.1 Mail system full 452 may not be able to delete material [RFC3463] (Standards Track) G. Vaudreuil IESG to make room for additional messages. This is useful only as a persistent transient error. The host on which the mailbox is resident is not accepting messages. System not Examples of such conditions include [RFC3463] (Standards Track); G. X.3.2 accepting network 453, 521 an imminent shutdown, excessive [RFC7504] (Standards Track) Vaudreuil, IESG messages load, or system maintenance. This is J. Klensin useful for both permanent and persistent transient errors. Selected features specified for the System not capable message are not supported by the X.3.3 of selected Not given destination system. This can occur [RFC3463] (Standards Track) G. Vaudreuil IESG features in gateways when features from one domain cannot be mapped onto the supported feature in another. The message is larger than Message too big for per-message size limit. This limit X.3.4 system 552, 554 may either be for physical or [RFC3463] (Standards Track) G. Vaudreuil IESG administrative reasons. This is useful only as a permanent error. System incorrectly The system is not configured in a X.3.5 configured Not given manner that will permit it to accept [RFC3463] (Standards Track) G. Vaudreuil IESG this message. The message was accepted for relay/delivery, but the requested priority (possibly the implied X.3.6 Requested priority 250 or 251 default) was not honoured. The human [RFC6710] (Standards Track) A. Melnikov IESG was changed readable text after the status code contains the new priority, followed by SP (space) and explanatory human readable text. Something went wrong with the Other or undefined networking, but it is not clear what X.4.0 network or routing Not given the problem is, or the problem [RFC3463] (Standards Track) G. Vaudreuil IESG status cannot be well expressed with any of the other provided detail codes. The outbound connection attempt was not answered, because either the X.4.1 No answer from host 451 remote system was busy, or was [RFC3463] (Standards Track) G. Vaudreuil IESG unable to take a call. This is useful only as a persistent transient error. The outbound connection was established, but was unable to complete the message transaction, X.4.2 Bad connection 421 either because of time-out, or [RFC3463] (Standards Track) G. Vaudreuil IESG inadequate connection quality. This is useful only as a persistent transient error. The network system was unable to forward the message, because a directory server was unavailable. X.4.3 Directory server 451, 550 This is useful only as a persistent [RFC3463] (Standards Track) G. Vaudreuil IESG failure transient error. The inability to connect to an Internet DNS server is one example of the directory server failure error. The mail system was unable to determine the next hop for the message because the necessary routing information was unavailable from the directory server. This is X.4.4 Unable to route Not given useful for both permanent and [RFC3463] (Standards Track) G. Vaudreuil IESG persistent transient errors. A DNS lookup returning only an SOA (Start of Administration) record for a domain name is one example of the unable to route error. The mail system was unable to Mail system deliver the message because the mail X.4.5 congestion 451 system was congested. This is useful [RFC3463] (Standards Track) G. Vaudreuil IESG only as a persistent transient error. A routing loop caused the message to be forwarded too many times, either X.4.6 Routing loop Not given because of incorrect routing tables [RFC3463] (Standards Track) G. Vaudreuil IESG detected or a user- forwarding loop. This is useful only as a persistent transient error. The message was considered too old by the rejecting system, either because it remained on that host too long or because the time-to-live X.4.7 Delivery time Not given value specified by the sender of the [RFC3463] (Standards Track) G. Vaudreuil IESG expired message was exceeded. If possible, the code for the actual problem found when delivery was attempted should be returned rather than this code. 220, 250, Something was wrong with the 251, 252, protocol necessary to deliver the Other or undefined 253, 451, message to the next hop and the X.5.0 protocol status 452, 454, problem cannot be well expressed [RFC3463] (Standards Track) G. Vaudreuil IESG 458, 459, with any of the other provided 501, 502, detail codes. 503, 554 430, 500, A mail transaction protocol command X.5.1 Invalid command 501, 503, was issued which was either out of [RFC3463] (Standards Track) G. Vaudreuil IESG 530, 550, sequence or unsupported. This is 554, 555 useful only as a permanent error. A mail transaction protocol command was issued which could not be X.5.2 Syntax error 500, 501, interpreted, either because the [RFC3463] (Standards Track) G. Vaudreuil IESG 502, 550, 555 syntax was wrong or the command is unrecognized. This is useful only as a permanent error. More recipients were specified for the message than could have been delivered by the protocol. This error should normally result in the X.5.3 Too many recipients 451 segmentation of the message into [RFC3463] (Standards Track) G. Vaudreuil IESG two, the remainder of the recipients to be delivered on a subsequent delivery attempt. It is included in this list in the event that such segmentation is not possible. A valid mail transaction protocol command was issued with invalid Invalid command 451, 501, arguments, either because the X.5.4 arguments 502, 503, arguments were out of range or [RFC3463] (Standards Track) G. Vaudreuil IESG 504, 550, 555 represented unrecognized features. This is useful only as a permanent error. A protocol version mis-match existed X.5.5 Wrong protocol Not given which could not be automatically [RFC3463] (Standards Track) G. Vaudreuil IESG version resolved by the communicating parties. This enhanced status code SHOULD be returned when the server fails the AUTH command due to the client Authentication sending a [BASE64] response which is R. X.5.6 Exchange line is 500 longer than the maximum buffer size [RFC4954] (Standards Track) Siemborski, IESG too long available for the currently selected A. Melnikov SASL mechanism. This is useful for both permanent and persistent transient errors. Something about the content of a Other or undefined message caused it to be considered X.6.0 media error Not given undeliverable and the problem cannot [RFC3463] (Standards Track) G. Vaudreuil IESG be well expressed with any of the other provided detail codes. The media of the message is not supported by either the delivery X.6.1 Media not supported Not given protocol or the next system in the [RFC3463] (Standards Track) G. Vaudreuil IESG forwarding path. This is useful only as a permanent error. The content of the message must be converted before it can be delivered Conversion required and such conversion is not X.6.2 and prohibited Not given permitted. Such prohibitions may be [RFC3463] (Standards Track) G. Vaudreuil IESG the expression of the sender in the message itself or the policy of the sending host. The message content must be converted in order to be forwarded but such conversion is not possible Conversion required or is not practical by a host in the X.6.3 but not supported 554 forwarding path. This condition may [RFC3463] (Standards Track) G. Vaudreuil IESG result when an ESMTP gateway supports 8bit transport but is not able to downgrade the message to 7 bit as required for the next hop. This is a warning sent to the sender when message delivery was successfully but when the delivery X.6.4 Conversion with 250 required a conversion in which some [RFC3463] (Standards Track) G. Vaudreuil IESG loss performed data was lost. This may also be a permanent error if the sender has indicated that conversion with loss is prohibited for the message. A conversion was required but was X.6.5 Conversion Failed Not given unsuccessful. This may be useful as [RFC3463] (Standards Track) G. Vaudreuil IESG a permanent or persistent temporary notification. The message content could not be X.6.6 Message content not 554 fetched from a remote system. This [RFC4468] (Standards Track) C. Newman IESG available may be useful as a permanent or persistent temporary notification. Non-ASCII addresses This indicates the reception of a X.6.7 not permitted for 553, 550 MAIL or RCPT command that non-ASCII [RFC6531] (Standards track) Jiankang YAO ima@ietf.org that addresses are not permitted sender/recipient UTF-8 string reply This indicates that a reply is required, but containing a UTF-8 string is X.6.8 not permitted by 252, 553, 550 required to show the mailbox name, [RFC6531] (Standards track) Jiankang YAO ima@ietf.org the SMTP client but that form of response is not permitted by the SMTP client. UTF-8 header message cannot be This indicates that transaction X.6.9 transferred to one 550 failed after the final "." of the [RFC6531] (Standards track) Jiankang YAO ima@ietf.org or more recipients, DATA command. so the message must be rejected X.6.10 This is a duplicate of X.6.8 and is [RFC6531] (Standards track) thus deprecated. Something related to security caused 220, 235, the message to be returned, and the 450, 454, problem cannot be well expressed X.7.0 Other or undefined 500, 501, with any of the other provided [RFC3463] (Standards Track) G. Vaudreuil IESG security status 503, 504, detail codes. This status code may 530, 535, 550 also be used when the condition cannot be further described because of security policies in force. The sender is not authorized to send to the destination. This can be the Delivery not 451, 454, result of per-host or per-recipient X.7.1 authorized, message 502, 503, filtering. This memo does not [RFC3463] (Standards Track) G. Vaudreuil IESG refused 533, 550, 551 discuss the merits of any such filtering, but provides a mechanism to report such. This is useful only as a permanent error. Mailing list The sender is not authorized to send X.7.2 expansion 550 a message to the intended mailing [RFC3463] (Standards Track) G. Vaudreuil IESG prohibited list. This is useful only as a permanent error. A conversion from one secure Security conversion messaging protocol to another was X.7.3 required but not Not given required for delivery and such [RFC3463] (Standards Track) G. Vaudreuil IESG possible conversion was not possible. This is useful only as a permanent error. A message contained security features such as secure X.7.4 Security features 504 authentication that could not be [RFC3463] (Standards Track) G. Vaudreuil IESG not supported supported on the delivery protocol. This is useful only as a permanent error. A transport system otherwise authorized to validate or decrypt a X.7.5 Cryptographic Not given message in transport was unable to [RFC3463] (Standards Track) G. Vaudreuil IESG failure do so because necessary information such as key was not available or such information was invalid. A transport system otherwise Cryptographic authorized to validate or decrypt a X.7.6 algorithm not Not given message was unable to do so because [RFC3463] (Standards Track) G. Vaudreuil IESG supported the necessary algorithm was not supported. A transport system otherwise authorized to validate a message was Message integrity unable to do so because the message X.7.7 failure Not given was corrupted or altered. This may [RFC3463] (Standards Track) G. Vaudreuil IESG be useful as a permanent, transient persistent, or successful delivery code. This response to the AUTH command indicates that the authentication failed due to invalid or R. X.7.8 Authentication 535, 554 insufficient authentication [RFC4954] (Standards Track) Siemborski, IESG credentials invalid credentials. In this case, the A. Melnikov client SHOULD ask the user to supply new credentials (such as by presenting a password dialog box). This response to the AUTH command Authentication indicates that the selected R. X.7.9 mechanism is too 534 authentication mechanism is weaker [RFC4954] (Standards Track) Siemborski, IESG weak than server policy permits for that A. Melnikov user. The client SHOULD retry with a new authentication mechanism. This indicates that external strong privacy layer is needed in order to use the requested authentication mechanism. This is primarily X.7.10 Encryption Needed 523 intended for use with clear text [RFC5248] (Best current practice) T. Hansen, IESG authentication mechanisms. A client J. Klensin which receives this may activate a security layer such as TLS prior to authenticating, or attempt to use a stronger mechanism. This response to the AUTH command indicates that the selected authentication mechanism may only be used when the underlying SMTP Encryption required connection is encrypted. Note that for requested this response code is documented R. X.7.11 authentication 524, 538 here for historical purposes only. [RFC4954] (Standards Track) Siemborski, IESG mechanism Modern implementations SHOULD NOT A. Melnikov advertise mechanisms that are not permitted due to lack of encryption, unless an encryption layer of sufficient strength is currently being employed. This response to the AUTH command indicates that the user needs to transition to the selected A password authentication mechanism. This is R. X.7.12 transition is 422, 432 typically done by authenticating [RFC4954] (Standards Track) Siemborski, IESG needed once using the [PLAIN] A. Melnikov authentication mechanism. The selected mechanism SHOULD then work for authentications in subsequent sessions. Sometimes a system administrator will have to disable a user's account (e.g., due to lack of payment, abuse, evidence of a break-in attempt, etc). This error code occurs after a successful authentication to a disabled User Account account. This informs the client T. Hansen, X.7.13 Disabled 525 that the failure is permanent until [RFC5248] (Best current practice) J. Klensin IESG the user contacts their system administrator to get the account re-enabled. It differs from a generic authentication failure where the client's best option is to present the passphrase entry dialog in case the user simply mistyped their passphrase. The submission server requires a configured trust relationship with a Trust relationship third-party server in order to T. Hansen, X.7.14 required 535, 554 access the message content. This [RFC5248] (Best current practice) J. Klensin IESG value replaces the prior use of X.7.8 for this error condition. thereby updating [RFC4468]. The specified priority level is below the lowest priority acceptable 450, 550 for the receiving SMTP server. This Priority Level is (other 4XX or condition might be temporary, for X.7.15 too low 5XX codes are example the server is operating in a [RFC6710] (Standards Track) A. Melnikov IESG allowed) mode where only higher priority messages are accepted for transfer and delivery, while lower priority messages are rejected. The message is too big for the 552 (other specified priority. This condition Message is too big 4XX or 5XX might be temporary, for example the X.7.16 for the specified codes are server is operating in a mode where [RFC6710] (Standards Track) A. Melnikov IESG priority allowed) only higher priority messages below certain size are accepted for transfer and delivery. This status code is returned when a message is received with a Require-Recipient-Valid-Since field X.7.17 Mailbox owner has 5XX or RRVS extension and the receiving [RFC7293] (Standards Track) M. Kucherawy IESG changed system is able to determine that the intended recipient mailbox has not been under continuous ownership since the specified date-time. This status code is returned when a message is received with a Require-Recipient-Valid-Since field X.7.18 Domain owner has 5XX or RRVS extension and the receiving [RFC7293] (Standards Track) M. Kucherawy IESG changed system wishes to disclose that the owner of the domain name of the recipient has changed since the specified date-time. This status code is returned when a message is received with a Require-Recipient-Valid-Since field or RRVS extension and the receiving X.7.19 RRVS test cannot be 5XX system cannot complete the requested [RFC7293] (Standards Track) M. Kucherawy IESG completed evaluation because the required timestamp was not recorded. The message originator needs to decide whether to reissue the message without RRVS protection. This status code is returned when a X.7.20 No passing DKIM 550 message did not contain any passing [RFC7372] (Standards Track); M. Kucherawy IESG signature found DKIM signatures. (This violates the [RFC6376] (Standards Track) advice of Section 6.1 of [RFC6376].) This status code is returned when a No acceptable DKIM message contains one or more passing [RFC7372] (Standards Track); X.7.21 signature found 550 DKIM signatures, but none are [RFC6376] (Standards Track) M. Kucherawy IESG acceptable. (This violates the advice of Section 6.1 of [RFC6376].) This status code is returned when a message contains one or more passing DKIM signatures, but none are No valid acceptable because none have an [RFC7372] (Standards Track); X.7.22 author-matched DKIM 550 identifier(s) that matches the [RFC6376] (Standards Track) M. Kucherawy IESG signature found author address(es) found in the From header field. This is a special case of X.7.21. (This violates the advice of Section 6.1 of [RFC6376].) This status code is returned when a message completed an SPF check that X.7.23 SPF validation 550 produced a "fail" result, contrary [RFC7372] (Standards Track); M. Kucherawy IESG failed to local policy requirements. Used [RFC7208] (Standards Track) in place of 5.7.1 as described in Section 8.4 of [RFC7208]. This status code is returned when evaluation of SPF relative to an X.7.24 SPF validation 451/550 arriving message resulted in an [RFC7372] (Standards Track); M. Kucherawy IESG error error. Used in place of 4.4.3 or [RFC7208] (Standards Track) 5.5.2 as described in Sections 8.6 and 8.7 of [RFC7208]. This status code is returned when an Reverse DNS SMTP client's IP address failed a [RFC7372], Section 3.3 (Standards X.7.25 validation failed 550 reverse DNS validation check, Track) M. Kucherawy IESG contrary to local policy requirements. This status code is returned when a Multiple message failed more than one message X.7.26 authentication 550 authentication check, contrary to [RFC7372] (Standards Track) M. Kucherawy IESG checks failed local policy requirements. The particular mechanisms that failed are not specified. This status code is returned when the associated sender address has a X.7.27 Sender address has 550 null MX, and the SMTP receiver is [RFC7505] (Standards Track) J. Levine, IESG null MX configured to reject mail from such M. Delany sender (e.g., because it could not return a DSN). The message appears to be part of a X.7.28 Mail flood detected . mail flood of similar abusive [draft-levine-mailbomb-header-00] J. Levine standards@taugh.com messages. X.7.29 ARC validation 550 This status code may be returned [RFC8617] K. Andersen IESG failure when a message fails ARC validation. This indicates that the message was not able to be forwarded because it X.7.30 REQUIRETLS support 550 was received with a REQUIRETLS [RFC8689] J. Fenton IESG required requirement and none of the SMTP servers to which the message should be forwarded provide this support. Licensing Terms