Internet Assigned Numbers Authority Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry Created 2008-05-29 Last Updated 2019-11-27 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. X.1.0 Other address Not given Something about the address specified [RFC3463] (Standards Track) G. Vaudreuil IESG status in the message caused this 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 portion [RFC3463] (Standards Track) G. Vaudreuil IESG mailbox address 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 Bad destination incapable of accepting mail. For X.1.2 system address Not given Internet mail names, this means the [RFC3463] (Standards Track) G. Vaudreuil IESG address portion to the right of the "@" is invalid for mail. This code is only useful for permanent failures. Bad destination The destination address was X.1.3 mailbox address 501 syntactically invalid. This can apply [RFC3463] (Standards Track) G. Vaudreuil IESG syntax to any field in the address. This code is only useful for permanent failures. The mailbox address as specified matches one or more recipients on the X.1.4 Destination mailbox Not given destination system. This may result if [RFC3463] (Standards Track) G. Vaudreuil IESG address ambiguous a heuristic address mapping algorithm is used to map the specified address to a local mailbox name. Destination address This mailbox address as specified was X.1.5 valid 250 valid. This status code should be used [RFC3463] (Standards Track) G. Vaudreuil IESG for positive delivery reports. The mailbox address provided was at Destination mailbox one time valid, but mail is no longer X.1.6 has moved, No Not given being accepted for that address. This [RFC3463] (Standards Track) G. Vaudreuil IESG forwarding address code is only useful for permanent failures. Bad sender's The sender's address was syntactically X.1.7 mailbox address Not given invalid. This can apply to any field [RFC3463] (Standards Track) G. Vaudreuil IESG syntax in the address. The sender's system specified in the address does not exist or is incapable X.1.8 Bad sender's system 451, 501 of accepting return mail. For domain [RFC3463] (Standards Track) G. Vaudreuil IESG address 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 the [RFC7505] (Standards Track); J. Levine, M. X.1.10 has null MX 556 associated address is marked as [RFC7504] (Standards Track) Delany, J. IESG invalid using a null MX. 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 address Mailing list and the mailing list was unable to be X.2.4 expansion problem 450, 452 expanded. This code may represent a [RFC3463] (Standards Track) G. Vaudreuil IESG 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 X.3.1 Mail system full 452 individual recipient may not be able [RFC3463] (Standards Track) G. Vaudreuil IESG to delete material 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 an [RFC3463] (Standards Track); G. Vaudreuil, X.3.2 accepting network 453, 521 imminent shutdown, excessive load, or [RFC7504] (Standards Track) J. Klensin IESG messages system maintenance. This is 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 in [RFC3463] (Standards Track) G. Vaudreuil IESG features gateways when features from one domain cannot be mapped onto the supported feature in another. The message is larger than per-message Message too big for size limit. This limit may either be X.3.4 system 552, 554 for physical or administrative [RFC3463] (Standards Track) G. Vaudreuil IESG 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 cannot [RFC3463] (Standards Track) G. Vaudreuil IESG status 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 unable [RFC3463] (Standards Track) G. Vaudreuil IESG 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. This X.4.3 Directory server 451, 550 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 useful for X.4.4 Unable to route Not given both permanent and persistent [RFC3463] (Standards Track) G. Vaudreuil IESG 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 deliver X.4.5 Mail system 451 the message because the mail system [RFC3463] (Standards Track) G. Vaudreuil IESG congestion was congested. This is useful 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 or [RFC3463] (Standards Track) G. Vaudreuil IESG detected 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 Delivery time because the time-to-live value X.4.7 expired Not given specified by the sender of the message [RFC3463] (Standards Track) G. Vaudreuil IESG was exceeded. If possible, the code for the actual problem found when delivery was attempted should be returned rather than this code. 220, 250, 251, 252, Something was wrong with the protocol Other or undefined 253, 451, necessary to deliver the message to X.5.0 protocol status 452, 454, the next hop and the problem cannot be [RFC3463] (Standards Track) G. Vaudreuil IESG 458, 459, well expressed with any of the other 501, 502, provided 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 syntax [RFC3463] (Standards Track) G. Vaudreuil IESG 502, 550, 555 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 segmentation of X.5.3 Too many recipients 451 the message into two, the remainder of [RFC3463] (Standards Track) G. Vaudreuil IESG 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. Wrong protocol A protocol version mis-match existed X.5.5 version Not given which could not be automatically [RFC3463] (Standards Track) G. Vaudreuil IESG resolved by the communicating parties. This enhanced status code SHOULD be returned when the server fails the AUTH command due to the client sending Authentication a [BASE64] response which is longer R. X.5.6 Exchange line is 500 than the maximum buffer size available [RFC4954] (Standards Track) Siemborski, IESG too long for the currently selected SASL A. Melnikov 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 permitted. X.6.2 and prohibited Not given Such prohibitions may be the [RFC3463] (Standards Track) G. Vaudreuil IESG 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 or is not Conversion required practical by a host in the forwarding X.6.3 but not supported 554 path. This condition may result when [RFC3463] (Standards Track) G. Vaudreuil IESG 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 required a X.6.4 Conversion with 250 conversion in which some data was [RFC3463] (Standards Track) G. Vaudreuil IESG loss performed 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 a [RFC3463] (Standards Track) G. Vaudreuil IESG permanent or persistent temporary notification. The message content could not be X.6.6 Message content not 554 fetched from a remote system. This may [RFC4468] (Standards Track) C. Newman IESG available be useful as a permanent or persistent temporary notification. Non-ASCII addresses This indicates the reception of a MAIL X.6.7 not permitted for 553, 550 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 containing is required, but a UTF-8 string is required to show the X.6.8 not permitted by 252, 553, 550 mailbox name, but that form of [RFC6531] (Standards track) Jiankang YAO ima@ietf.org the SMTP client response is not permitted by the SMTP client. UTF-8 header message cannot be This indicates that transaction failed X.6.9 transferred to one 550 after the final "." of the DATA [RFC6531] (Standards track) Jiankang YAO ima@ietf.org or more recipients, 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 with X.7.0 Other or undefined 500, 501, any of the other provided detail [RFC3463] (Standards Track) G. Vaudreuil IESG security status 503, 504, codes. This status code may also be 530, 535, 550 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 discuss [RFC3463] (Standards Track) G. Vaudreuil IESG refused 533, 550, 551 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 a X.7.2 expansion 550 message to the intended mailing list. [RFC3463] (Standards Track) G. Vaudreuil IESG prohibited This is useful only as a permanent error. A conversion from one secure messaging Security conversion protocol to another was required for X.7.3 required but not Not given delivery and such conversion was not [RFC3463] (Standards Track) G. Vaudreuil IESG possible possible. This is useful only as a permanent error. A message contained security features Security features such as secure authentication that X.7.4 not supported 504 could not be supported on the delivery [RFC3463] (Standards Track) G. Vaudreuil IESG 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 do [RFC3463] (Standards Track) G. Vaudreuil IESG failure 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 be [RFC3463] (Standards Track) G. Vaudreuil IESG useful as a permanent, transient persistent, or successful delivery code. This response to the AUTH command indicates that the authentication Authentication failed due to invalid or insufficient R. X.7.8 credentials invalid 535, 554 authentication credentials. In this [RFC4954] (Standards Track) Siemborski, IESG case, the client SHOULD ask the user A. Melnikov 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 intended X.7.10 Encryption Needed 523 for use with clear text authentication [RFC5248] (Best current T. Hansen, J. IESG mechanisms. A client which receives practice) Klensin 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 R. X.7.11 for requested 524, 538 this response code is documented here [RFC4954] (Standards Track) Siemborski, IESG authentication for historical purposes only. Modern A. Melnikov mechanism implementations SHOULD NOT 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 once [RFC4954] (Standards Track) Siemborski, IESG needed using the [PLAIN] authentication A. Melnikov 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 User Account disabled account. This informs the [RFC5248] (Best current T. Hansen, J. X.7.13 Disabled 525 client that the failure is permanent practice) Klensin IESG until 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 access [RFC5248] (Best current T. Hansen, J. X.7.14 required 535, 554 the message content. This value practice) Klensin IESG 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 for the 450, 550 receiving SMTP server. This condition Priority Level is (other 4XX or might be temporary, for example the X.7.15 too low 5XX codes are server is operating in a mode where [RFC6710] (Standards Track) A. Melnikov IESG allowed) 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 or X.7.17 Mailbox owner has 5XX 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 or X.7.18 Domain owner has 5XX 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 author [RFC6376] (Standards Track) M. Kucherawy IESG signature found 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 to [RFC7372] (Standards Track); M. Kucherawy IESG failed local policy requirements. Used in [RFC7208] (Standards Track) 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 error. [RFC7372] (Standards Track); M. Kucherawy IESG error Used in place of 4.4.3 or 5.5.2 as [RFC7208] (Standards Track) described in Sections 8.6 and 8.7 of [RFC7208]. This status code is returned when an X.7.25 Reverse DNS 550 SMTP client's IP address failed a [RFC7372], Section 3.3 M. Kucherawy IESG validation failed reverse DNS validation check, contrary (Standards Track) 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 null X.7.27 Sender address has 550 MX, and the SMTP receiver is [RFC7505] (Standards Track) J. Levine, M. IESG null MX configured to reject mail from such 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] J. Levine standards@taugh.com messages. X.7.29 ARC validation 550 This status code may be returned when [RFC8617] K. Andersen IESG failure 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