Internet Assigned Numbers Authority • Domains • Protocols • Numbers • About 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] TXT 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 [RFC 5248] Available Formats [IMG] CSV Code Summary Description Reference Submitter Change Controller Success specifies that the DSN is reporting a positive delivery [RFC 3463] (Standards 2.XXX.YYY Success 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 sent Persistent Transient is valid, but persistence of some temporary condition has caused [RFC 3463] (Standards 4.XXX.YYY Failure abandonment or delay of attempts to send the message. If this code track) G. Vaudreuil IESG accompanies a delivery failure report, sending in the future may be successful. A permanent failure is one which is not likely to be resolved by [RFC 3463] (Standards 5.XXX.YYY Permanent Failure resending the message in the current form. Some change to the track) G. Vaudreuil IESG message or the destination must be made for successful delivery. Subject Sub-Codes Registration Procedure(s) Specification Required Expert(s) Chris Newman Reference [RFC 5248] Available Formats [IMG] CSV Code Summary Description Reference Submitter Change Controller X.0.YYY Other or Undefined Status There is no additional subject information available. [RFC 3463] (Standards G. Vaudreuil IESG track) The address status reports on the originator or destination [RFC 3463] (Standards X.1.YYY Addressing Status address. It may include address syntax or validity. These track) G. Vaudreuil IESG errors can generally be corrected by the sender and retried. Mailbox status indicates that something having to do with the [RFC 3463] (Standards X.2.YYY Mailbox Status mailbox has caused this DSN. Mailbox issues are assumed to be track) G. Vaudreuil IESG under the general control of the recipient. Mail system status indicates that something having to do with X.3.YYY Mail System Status the destination system has caused this DSN. System issues are [RFC 3463] (Standards G. Vaudreuil IESG assumed to be under the general control of the destination track) system administrator. The networking or routing codes report status about the Network and Routing delivery system itself. These system components include any [RFC 3463] (Standards X.4.YYY Status necessary infrastructure such as directory and routing track) G. Vaudreuil IESG services. Network issues are assumed to be under the 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 include [RFC 3463] (Standards G. Vaudreuil IESG Status the full range of problems resulting from implementation errors track) or an unreliable connection. The message content or media status codes report failures involving the content of the message. These codes report X.6.YYY Message Content or Media failures due to translation, transcoding, or otherwise [RFC 3463] (Standards G. Vaudreuil IESG Status unsupported message media. Message content or media issues are track) 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 status issues are [RFC 3463] (Standards X.7.YYY Security or Policy Status assumed to be under the control of either or both the sender track) G. Vaudreuil IESG and recipient. Both the sender and 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 [RFC 5248] 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 [RFC 3463] (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 status Not given Something about the address specified [RFC 3463] (Standards Track) G. Vaudreuil IESG 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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (Standards Track) G. Vaudreuil IESG forwarding address code is only useful for permanent failures. Bad sender's mailbox The sender's address was syntactically X.1.7 address syntax Not given invalid. This can apply to any field [RFC 3463] (Standards Track) G. Vaudreuil IESG 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 [RFC 3463] (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 mailer Not given relayed to a system that does not [RFC 3886] (Standards Track) E. Allman IESG speak this protocol; no further information can be provided. Recipient address This status code is returned when the [RFC 7505] (Standards Track); [RFC J. Levine, M. X.1.10 has null MX 556 associated address is marked as 7504] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 length limit has been exceeded. This Message length status code should be used when the X.2.3 exceeds 552 per-mailbox message length limit is [RFC 3463] (Standards Track) G. Vaudreuil IESG administrative 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 [RFC 3463] (Standards Track) G. Vaudreuil IESG permanent failure or a persistent transient failure. The destination system exists and X.3.0 Other or undefined 221, 250, 421, normally accepts mail, but something [RFC 3463] (Standards Track) G. Vaudreuil IESG mail system status 451, 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 [RFC 3463] (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 accepting Examples of such conditions include an [RFC 3463] (Standards Track); [RFC G. Vaudreuil, X.3.2 network messages 453, 521 imminent shutdown, excessive load, or 7504] (Standards Track) J. Klensin IESG system maintenance. This is useful for both permanent and persistent transient errors. Selected features specified for the message are not supported by the X.3.3 System not capable Not given destination system. This can occur in [RFC 3463] (Standards Track) G. Vaudreuil IESG of selected 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 [RFC 3463] (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 [RFC 3463] (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 [RFC 6710] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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, Something was wrong with the protocol Other or undefined 252, 253, 451, necessary to deliver the message to X.5.0 protocol status 452, 454, 458, the next hop and the problem cannot be [RFC 3463] (Standards Track) G. Vaudreuil IESG 459, 501, 502, well expressed with any of the other 503, 554 provided detail codes. 430, 500, 501, A mail transaction protocol command X.5.1 Invalid command 503, 530, 550, was issued which was either out of [RFC 3463] (Standards Track) G. Vaudreuil IESG 554, 555 sequence or unsupported. This is useful only as a permanent error. A mail transaction protocol command was issued which could not be X.5.2 Syntax error 500, 501, 502, interpreted, either because the syntax [RFC 3463] (Standards Track) G. Vaudreuil IESG 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 [RFC 3463] (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, 502, arguments, either because the X.5.4 arguments 503, 504, 550, arguments were out of range or [RFC 3463] (Standards Track) G. Vaudreuil IESG 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 [RFC 3463] (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 too 500 than the maximum buffer size available [RFC 4954] (Standards Track) Siemborski, IESG 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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 loss 250 conversion in which some data was [RFC 3463] (Standards Track) G. Vaudreuil IESG 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 [RFC 3463] (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 [RFC 4468] (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 [RFC 6531] (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 not a UTF-8 string is required to show the X.6.8 permitted by the 252, 553, 550 mailbox name, but that form of [RFC 6531] (Standards track) Jiankang YAO ima@ietf.org 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 [RFC 6531] (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 [RFC 6531] (Standards track) thus deprecated. Something related to security caused the message to be returned, and the 220, 235, 450, problem cannot be well expressed with X.7.0 Other or undefined 454, 500, 501, any of the other provided detail [RFC 3463] (Standards Track) G. Vaudreuil IESG security status 503, 504, 530, codes. This status code may also be 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, 502, result of per-host or per-recipient X.7.1 authorized, message 503, 533, 550, filtering. This memo does not discuss [RFC 3463] (Standards Track) G. Vaudreuil IESG refused 551 the merits of any such filtering, but provides a mechanism to report such. This is useful only as a permanent error. The sender is not authorized to send a X.7.2 Mailing list 550 message to the intended mailing list. [RFC 3463] (Standards Track) G. Vaudreuil IESG expansion 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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 3463] (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 [RFC 4954] (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 [RFC 4954] (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 [RFC 5248] (Best current practice) T. Hansen, J. IESG mechanisms. A client which receives 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 [RFC 4954] (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 needed 422, 432 typically done by authenticating once [RFC 4954] (Standards Track) Siemborski, IESG 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 T. Hansen, J. X.7.13 Disabled 525 client that the failure is permanent [RFC 5248] (Best current 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 T. Hansen, J. X.7.14 required 535, 554 the message content. This value [RFC 5248] (Best current practice) Klensin IESG replaces the prior use of X.7.8 for this error condition. thereby updating [RFC 4468]. 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 [RFC 6710] (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 specified priority. This condition Message is too big 552 (other 4XX might be temporary, for example the X.7.16 for the specified or 5XX codes server is operating in a mode where [RFC 6710] (Standards Track) A. Melnikov IESG priority are 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 [RFC 7293] (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 [RFC 7293] (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 [RFC 7293] (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 [RFC 7372] (Standards Track); [RFC M. Kucherawy IESG signature found DKIM signatures. (This violates the 6376] (Standards Track) advice of [Section 6.1 of RFC 6376].) This status code is returned when a No acceptable DKIM message contains one or more passing [RFC 7372] (Standards Track); [RFC X.7.21 signature found 550 DKIM signatures, but none are 6376] (Standards Track) M. Kucherawy IESG acceptable. (This violates the advice of [Section 6.1 of RFC 6376].) 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 [RFC 7372] (Standards Track); [RFC X.7.22 author-matched DKIM 550 identifier(s) that matches the author 6376] (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 RFC 6376].) 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 [RFC 7372] (Standards Track); [RFC M. Kucherawy IESG failed local policy requirements. Used in 7208] (Standards Track) place of 5.7.1 as described in [Section 8.4 of RFC 7208]. This status code is returned when evaluation of SPF relative to an X.7.24 SPF validation error 451/550 arriving message resulted in an error. [RFC 7372] (Standards Track); [RFC M. Kucherawy IESG Used in place of 4.4.3 or 5.5.2 as 7208] (Standards Track) described in [Sections 8.6, 8.7 of RFC 7208]. This status code is returned when an X.7.25 Reverse DNS 550 SMTP client's IP address failed a [RFC 7372, Section 3.3] (Standards M. Kucherawy IESG validation failed reverse DNS validation check, contrary 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 [RFC 7372] (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 [RFC 7505] (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-00] J. Levine standards@taugh.com messages. X.7.29 ARC validation 550 This status code may be returned when [RFC 8617] 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 [RFC 8689] J. Fenton IESG required requirement and none of the SMTP servers to which the message should be forwarded provide this support. Licensing Terms